#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ll long long
#define ld long double
#define el "\n"
#define matrix vector<vector<int>>
#define pt complex<ld>
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define ordered_multiset tree<ll, null_type,less_equal<ll>, rb_tree_tag, tree_order_statistics_node_update>
using namespace __gnu_pbds;
using namespace std;
const ll N = 1e5 + 5, LOG = 20, INF = 1e18;
const ld pi = acos(-1);
const ll mod = 1e9 + 7;
const ld eps = 1e-12;
int dx[] = {0, -1, 0, 1, -1, 1, -1, 1};
int dy[] = {-1, 0, 1, 0, 1, -1, -1, 1};
ll n, m, k, x, y;
string s[N];
int has[N];
int least[(1 << 27)];

bool mnwr(int bit, int mask) {
    if ((1 << bit) & mask) {
        return 1;
    }
    return 0;
}

int nwr(int bit, int mask) {
    return mask + (1 << bit);
}

int getbit(int mask) {
    return (mask & -(mask - 1));
}

bool solve(vector<int> &idx, int done) {
    if (idx.size() == 1) {
        return 1;
    }
    int next = has[idx[0]] ^ done;
    for (auto j: idx) {
        next &= has[j];
    }
    if (next == 0) {
        return 0;
    }
    char key = least[getbit(next)] + 'a';
    done = nwr(least[getbit(next)], done);
    map<vector<int>, vector<int>> ne;
    for (auto j: idx) {
        vector<int> v;
        for (int i = 0; i < s[j].size(); i++) {
            if (s[j][i] == key) {
                v.push_back(i);
            }
        }
        ne[v].push_back(j);
    }
    bool ans = 1;
    for (auto j: ne) {
        ans &= solve(j.second, done);
    }
    return ans;
}

void dowork() {
    vector<int> idx;
    cin >> n >> n;
    for (int i = 1; i <= n; i++) {
        cin >> s[i];
        for (auto j: s[i]) {
            if (mnwr(j - 'a', has[i]) == 0) {
                has[i] = nwr(j - 'a', has[i]);
            }
        }
        idx.push_back(i);
    }
    if (solve(idx, 0)) {
        cout << "YES" << el;
    } else {
        cout << "NO" << el;
    }
}

signed main() {
    //fast
    //freopen("lis.in", "r", stdin);
    //freopen("bitwise.out", "w", stdout);
    int t = 1;
    int id = 0;
    for (int i = 1; i < (1LL << 27); i *= 2) {
        least[i] = id;
        id++;
    }
    //cin >> t;
    for (int i = 1; i <= t; i++) {
        dowork();
    }
    return 0;
}