#ifndef SorahISA
#define SorahISA
#include SorahISA __FILE__ SorahISA

const int mod = 1'000'000'000'000'037;
const int P = 257;

void solve() {
    int L, N; cin >> L >> N;
    
    vector<string> strs(N);
    for (string &str : strs) cin >> str;
    
    vector<array<int, 26>> hashes(N);
    for (int i = 0; i < N; ++i) {
        for (int c = 0; c < 26; ++c) hashes[i][c] = 0;
        for (int p = 0; p < L; ++p) {
            int &meow = hashes[i][strs[i][p] - 'a'];
            meow = (meow * P + p) % mod + 1;
        }
    }
    
    using IT = vector<int>::iterator;
    
    bool flag = true;
    function<void(IT, IT, int)> recur = [&](IT it_bg, IT it_ed, int ban) {
        int n = distance(it_bg, it_ed), key = -1;
        if (n <= 1) return;
        
        vector<int> occ(26, 0);
        for (int c = 0; c < 26; ++c) { if (ban >> c & 1) occ[c] = -1; }
        for (IT it = it_bg; it != it_ed; it = next(it)) {
            for (int c = 0; c < 26; ++c) { if (hashes[*it][c] and ++occ[c] == n) key = c; }
        }
        if (key == -1) { flag = false; return; }
        sort(it_bg, it_ed, [&](int x, int y) { return hashes[x][key] < hashes[y][key]; });
        
        int new_ban = ban | (1 << key);
        int lst_hash = hashes[*it_bg][key];
        for (IT it = it_bg; it != it_ed; it = next(it)) {
            if (hashes[*it][key] != lst_hash) {
                recur(it_bg, it, new_ban), it_bg = it;
                lst_hash = hashes[*it][key];
            }
        }
        recur(it_bg, it_ed, new_ban);
    };
    vector<int> id(N); iota(ALL(id), 0);
    recur(ALL(id), 0);
    
    cout << (flag ? "YES" : "NO") << "\n";
}

int32_t main() {
    fastIO();
    
    int t = 1; // cin >> t;
    for (int _ = 1; _ <= t; ++_) {
        // cout << "Case #" << _ << ": ";
        solve();
    }
    
    return 0;
}

#else

#ifdef local
#define _GLIBCXX_DEBUG 1
#endif
#pragma GCC optimize("Ofast", "unroll-loops")
#include <bits/stdc++.h>
using namespace std;
// #include <bits/extc++.h>
// #include <tr2/dynamic_bitset>

using i64 = long long;
using i128 = __int128;
#define int i64
using f80 = long double;
using f128 = __float128;
#define double f80
using pii = pair<int, int>;
template <typename T> using Prior = std::priority_queue<T>;
template <typename T> using prior = std::priority_queue<T, vector<T>, greater<T>>;

// #define X first
// #define Y second
#define eb emplace_back
#define ef emplace_front
#define ee emplace
#define pb pop_back
#define pf pop_front
#define ALL(x) begin(x), end(x)
#define RALL(x) rbegin(x), rend(x)
#define SZ(x) ((int)(x).size())

// template <size_t D, typename T> struct Vec : vector<Vec<D-1, T>> {
//     static_assert(D >= 1, "Vector dimension must be greater than zero!");
//     template <typename... Args> Vec(int n = 0, Args... args) : vector<Vec<D-1, T>>(n, Vec<D-1, T>(args...)) {}
// };

// template <typename T> struct Vec<1, T> : vector<T> {
//     Vec(int n = 0, const T& val = T()) : vector<T>(n, val) {}
// };

#ifdef local
#define fastIO() void()
#define debug(...) \
    _color.emplace_back("\u001b[31m"), \
    fprintf(stderr, "%sAt [%s], line %d: (%s) = ", _color.back().c_str(), __FUNCTION__, __LINE__, #__VA_ARGS__), \
    _do(__VA_ARGS__), _color.pop_back(), \
    fprintf(stderr, "%s", _color.back().c_str())
#define print(...) \
    fprintf(stdout, "%s", "\u001b[36m"), \
    _P(__VA_ARGS__), \
    fprintf(stdout, "%s", "\u001b[0m")

deque<string> _color{"\u001b[0m"};

template <typename T> concept is_string = is_same_v<T, string&> or is_same_v<T, const string&>;
template <typename T> concept is_iterable = requires (T _t) { begin(_t); };

template <typename T> inline void _print_err(T &&_t);
template <typename T> inline void _print_err(T &&_t) requires is_iterable<T> and (not is_string<T>);
template <size_t I, typename ...U> inline typename enable_if<I == sizeof...(U), void>::type _print_err(const tuple<U...> &);
template <size_t I, typename ...U> inline typename enable_if<I <  sizeof...(U), void>::type _print_err(const tuple<U...> &_t);
template <size_t I, typename ...U> inline typename enable_if<I == sizeof...(U), void>::type _print_err(tuple<U...> &);
template <size_t I, typename ...U> inline typename enable_if<I <  sizeof...(U), void>::type _print_err(tuple<U...> &_t);
template <typename T, typename U> ostream& operator << (ostream &os, const pair<T, U> &_tu);

inline void _do() { cerr << "\n"; }
template <typename T> inline void _do(T &&_t) { _print_err(_t), cerr << "\n"; }
template <typename T, typename ...U> inline void _do(T &&_t, U &&..._u) { _print_err(_t), cerr << ", ", _do(_u...); }
#else
#define fastIO() cin.tie(0)->sync_with_stdio(0)
#define debug(...) void()
#define print(...) _P(__VA_ARGS__)
#endif

inline void _P() { cout << "\n"; }
template <typename T> inline void _P(T &&_t) { cout << _t << "\n"; }
template <typename T, typename ...U> inline void _P(T &&_t, U &&..._u) { cout << _t << " ", _P(_u...); }

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

inline int getRand(int L, int R) {
    if (L > R) swap(L, R);
    return (int)(rng() % ((uint64_t)R - L + 1) + L);
}

template <typename T, typename U> bool chmin(T &lhs, U rhs) { return lhs > rhs ? lhs = rhs, 1 : 0; }
template <typename T, typename U> bool chmax(T &lhs, U rhs) { return lhs < rhs ? lhs = rhs, 1 : 0; }

/// below are Fast I/O and _print_err templates ///

/*
/// Fast I/O by FHVirus ///
/// https://fhvirus.github.io/blog/2020/fhvirus-io/ ///

#include <unistd.h>

const int S = 65536;

int OP = 0;
char OB[S];

inline char RC() {
    static char buf[S], *p = buf, *q = buf;
    return p == q and (q = (p = buf) + read(0, buf, S)) == buf ? -1 : *p++;
}

inline int RI() {
    static char c;
    int a;
    while (((c = RC()) < '0' or c > '9') and c != '-' and c != -1);
    if (c == '-') {
        a = 0;
        while ((c = RC()) >= '0' and c <= '9') a *= 10, a -= c ^ '0';
    }
    else {
        a = c ^ '0';
        while ((c = RC()) >= '0' and c <= '9') a *= 10, a += c ^ '0';
    }
    return a;
}

inline void WI(int n, char c = '\n') {
    static char buf[20], p;
    if (n == 0) OB[OP++] = '0';
    p = 0;
    if (n < 0) {
        OB[OP++] = '-';
        while (n) buf[p++] = '0' - (n % 10), n /= 10;
    }
    else {
        while (n) buf[p++] = '0' + (n % 10), n /= 10;
    }
    for (--p; p >= 0; --p) OB[OP++] = buf[p];
    OB[OP++] = c;
    if (OP > S-20) write(1, OB, OP), OP = 0;
}

/// Fast I/O by FHVirus ///
/// https://fhvirus.github.io/blog/2020/fhvirus-io/ ///
*/

#ifdef local

template <typename T> inline void _print_err(T &&_t) { cerr << _t; }

template <typename T> inline void _print_err(T &&_t) requires is_iterable<T> and (not is_string<T>) {
    _color.emplace_back(_color.back()), ++_color.back()[3];
    cerr << _color.back() << "[";
    for (bool _first = true; auto &_x : _t) {
        if (!_first) cerr << ", ";
        _print_err(_x), _first = false;
    }
    cerr << "]" << (_color.pop_back(), _color.back());
}

template <typename T, typename U> ostream& operator << (ostream &os, const pair<T, U> &_tu) {
    _color.emplace_back(_color.back()), ++_color.back()[3];
    cerr << _color.back() << "(";
    _print_err(_tu.first), cerr << ", ", _print_err(_tu.second);
    cerr << ")" << (_color.pop_back(), _color.back());
    return os;
}

template <size_t I = 0, typename ...U> inline typename enable_if<I == sizeof...(U), void>::type _print_err(const tuple<U...> &) {
    cerr << ")" << (_color.pop_back(), _color.back());
}

template <size_t I = 0, typename ...U> inline typename enable_if<I <  sizeof...(U), void>::type _print_err(const tuple<U...> &_t) {
    if (!I) {
        _color.emplace_back(_color.back()), ++_color.back()[3];
        cerr << _color.back();
    }
    cerr << (I ? ", " : "("), _print_err(get<I>(_t)), _print_err<I+1, U...>(_t);
}

template <size_t I = 0, typename ...U> inline typename enable_if<I == sizeof...(U), void>::type _print_err(tuple<U...> &) {
    cerr << ")" << (_color.pop_back(), _color.back());
}

template <size_t I = 0, typename ...U> inline typename enable_if<I <  sizeof...(U), void>::type _print_err(tuple<U...> &_t) {
    if (!I) {
        _color.emplace_back(_color.back()), ++_color.back()[3];
        cerr << _color.back();
    }
    cerr << (I ? ", " : "("), _print_err(get<I>(_t)), _print_err<I+1, U...>(_t);
}

#endif

#endif

/**
 *                                                                                                                 
 *                                                                                                                 
 *                                                                                                                 
 *                            iiiiii         iiiiiiiiii       iiiiiiiiiiiiii                                       
 *                       iiiiiiiiiiiii   iiiiiii    iiii    iiiiiiiiiiiiiii                          ii   iiii     
 *                    iiiiiiii     iiiiiiiii         iiii       iiii iii              iii          iiiiiiiiii      
 *                 iiiiiii          iiiiii           iiii    iiii   ii           iiiiiiiiii      iiii iiii         
 *               iiiiii            iiiii             iiii iiii        iii      iiii    iiiiiiiiiiiiiiiii  ii       
 *             iiiiii            iiiiiii            iiiiiii       iiiiiiii   iii    iiiiiiiiiiiiii iii  iiii       
 *           iiiiii             iiiiiii            iiiii   ii   iiii       iiiiiiiiiii iiii  iii iiii iiii      iii
 *          iiiii              iiiiiiii       ii        iiiii iiii    iiiiiiiii        iii iii  iii  iii  ii  iiii 
 *        iiiiii              iiiiiiii      iiiii     iiiii iiiiiiiiiiiiiiii         iii  iii  ii  iii  iii iiii   
 *       iiiii                 iiiiii     iiii     iiiiii iiiiiii    iii iii       iiii  ii   i   ii  iii  iii     
 *     iiiiii                            iiii  iiiiiiiiiiiiiii       iii iiii   iiiii  iii  ii  iii  iii  ii       
 *    iiiii                              iiiiiiii iiiiiiiiii       iiii   iiiiiiiii            ii  iii  ii         
 *   iiiii                                     iiiiii  iiii      iiiii              iii      ii   ii  i            
 * iiiiii                                  iiiiiiii   iiiii    iiiii                        ii  ii   ii            
 * iiiii                                iiii  iiii    iiiiiiiiiiii                             ii                  
 *  iii                              iiii   iiii       iiiiiiii                                                    
 *                                iiiii   iiii                                                                     
 *                              iiii     iiii                                                                      
 *                            iiii    iiiii                                                                        
 *                          iii     iiiii                                                                          
 *                        iii     iiiii                                                                            
 *                       iii   iiiiii                                                                              
 *                       iiiiiiiii                                                                                 
 *                       iiiiii                                                                                    
 *                                                                                                                 
 *                                                                                                                 
 *                                                                                                                 
**/