#include <bits/stdc++.h>

#define el '\n'

typedef long long ll;
typedef long double ld;

#define Beevo ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

using namespace std;

const int N = 1e6 + 5;

map<char, vector<int>> indices[N];

bool solve(vector<int> v) {
    if (v.size() <= 1)
        return 1;

    for (char c = 'a'; c <= 'z'; c++) {
        int cnt = 0;
        for (auto &i: v)
            cnt += indices[i].count(c);

        if (cnt == v.size()) {
            map<vector<int>, vector<int>> mp;
            for (auto &i: v)
                mp[indices[i][c]].push_back(i), indices[i].erase(c);

            bool ret = 1;
            for (auto &i: mp)
                ret &= solve(i.second);

            return ret;
        }
    }

    return 0;
}

void testCase() {
    int l, n;
    cin >> l >> n;

    string s;
    for (int i = 0; i < n; i++) {
        cin >> s;

        for (char c = 'a'; c <= 'z'; c++) {
            for (int j = 0; j < l; j++) {
                if (s[j] == c)
                    indices[i][c].push_back(j);
            }
        }
    }

    vector<int> v(n);

    iota(v.begin(), v.end(), 0);

    cout << (solve(v) ? "YES" : "NO");
}

signed main() {
    Beevo

    int t = 1;
//    cin >> t;

    while (t--)
        testCase();

    return 0;
}