#include <bits/stdc++.h>

using namespace std;

#define int long long
#define P pair<int, int>
const P inf = {-1, -1};
const int B1 = 3, B2 = 5, mod1 = 1e9 + 7, mod2 = 2e9 + 11;

void solve() {
    int m, n;
    cin >> m >> n;
    vector<string> a(n);
    for (auto &i: a) cin >> i;
    vector<vector<pair<int, int>>> hash(n, vector<pair<int, int>>(26, inf));
    int id = 0;
    queue<vector<vector<P>>> q;
    q.push({});
    auto &cur = q.front();
    for (auto &s: a) {
        for (int l = 0; l < 26; ++l) {
            auto &[h1, h2] = hash[id][l];
            bool f = false;
            for (int i = 0; i < m; ++i) {
                h1 = h1 * B1 % mod1;
                h2 = h2 * B2 % mod2;
                if (s[i] == (l + 'a')) {
                    f = true;
                    h1 = (h1 + 2) % mod1;
                    h2 = (h2 + 2) % mod2;
                } else {
                    h1 = (h1 + 1) % mod1;
                    h2 = (h2 + 1) % mod2;
                }
            }
            if (!f)
                tie(h1, h2) = inf;
        }
        cur.emplace_back(hash[id++]);
    }

    while (!q.empty()) {
        auto &s = q.front();
        int nl = -1;
        for (int l = 0; l < 26; ++l) {
            bool f = true;
            for (auto &it: s)
                if (it[l] == inf) f = false;
            if (f) {
                nl = l;
                break;
            }
        }
        if (nl == -1) {
            cout << "NO\n";
            return;
        }
        map<P, int> mp;
        vector<vector<vector<P>>> v;
        for (auto it: s) {
            if (!mp.count(it[nl])) {
                mp[it[nl]] = v.size();
                it[nl] = inf;
                v.push_back({it});
            } else {
                int id = mp[it[nl]];
                it[nl] = inf;
                v[id].push_back(it);
            }
        }
        q.pop();
        for (auto &i: v)
            if (i.size() > 1) q.push(i);
    }
    cout << "YES\n";
}

int32_t main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int tc = 1;
//    cin >> tc;
    for (int c = 1; c <= tc; c++) {
        solve();
//        cout << '\n';
    }
}