#include <bits/stdc++.h>
using namespace std;

typedef long long ll ;

int L, N;

bool G(vector<string>& s) {
    int n = (int)s.size();
    if (n == 1) { return true; }
    vector<int> F(26, 0);
    for (int i = 0; i < n; ++i) {
        vector<bool> f(26, 0);
        for (char c : s[i]) {
            f[c - 'a'] = 1;
        }
        for (int j = 0; j < 26; ++j) {
            F[j] += f[j];
        }
    }
    map<string, vector<int>> Groups;
    for (int i = 0; i < n; ++i) {
        string T = "";
        for (char c : s[i]) {
            if (F[c - 'a'] == n) {
                T += c;
            } else {
                T += '#';
            }
        }
        Groups[T].push_back(i);
    }
    if ((int)Groups.size() == 1) { return false; }
    bool ok = true;
    for (auto [_, a] : Groups) {
        vector<string> new_s;
        for (int j : a) {
            new_s.push_back(s[j]);
        }
        ok = ok && G(new_s);
    }
    return ok;
}

int main() {
    cin.tie(0);cin.sync_with_stdio(0);
    cout.tie(0);cout.sync_with_stdio(0);
    cin >> L >> N;
    vector<string> S(N);
    for (int i = 0; i < N; ++i) {
        cin >> S[i];
    }
    cout << (G(S) ? "YES" : "NO");
}