///Enta etfsh5t nseet el rank

#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,fma")

#include "bits/stdc++.h"
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update

using namespace std;
using namespace __gnu_pbds;

template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,
        tree_order_statistics_node_update>;
#define pb push_back
#define F first
#define S second
#define f(i, a, b) for (int i = a; i < b; i++)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define sz(x) (int)(x).size()
#define mp(x, y) make_pair(x, y)
#define popCnt(x) (__builtin_popcountll(x))
#define int ll

using ll = long long;
using ull = unsigned long long;
using uint = uint32_t;
using ii = pair<int, int>;

const int N = 1e5 + 6, A = 26, LG = 18, MOD = 1e9 + 7;
const long double PI = acos(-1);
const long double EPS = 1e-9;
const ll INF = 1e15;

int getMask(string s) {
    int ret = 0;
    for (char c: s)
        ret |= (1 << (c - 'a'));
    return ret;
}

int mask[N];
string str[N];

bool solve(vector<int> vec) {
    if (vec.size() == 1)
        return true;
    int totMask = (1 << 26) - 1;
    for (auto x: vec)totMask &= mask[x];
    if (totMask == 0)
        return false;
    map<string, vector<int>> mp;
    for (auto i: vec) {
        string cur = "";
        for (auto c: str[i])
            if (totMask >> (c - 'a') & 1)
                cur.push_back(c);
            else
                cur.push_back('?');
        mp[cur].push_back(i);
    }
    if (mp.size() == 1)
        return false;
    for (auto [trash, vec]: mp)
        if (!solve(vec))
            return false;
    return true;
}

void doWork() {
    int n;
    cin >> n >> n;
    vector<int> vec;
    f(i, 0, n) {
        cin >> str[i];
        vec.push_back(i);
        mask[i] = getMask(str[i]);
    }
    if (solve(vec))
        cout << "YES\n";
    else
        cout << "NO\n";

}


int32_t main() {
#ifdef ONLINE_JUDGE
    ios_base::sync_with_stdio(0);
    cin.tie(0);
#endif // ONLINE_JUDGE
    int t = 1;
//    cin >> t;
    while (t--)
        doWork();

    return 0;
}