#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define EPS 1e-9
#define all(x) x.begin(),x.end()
#define allr(x) x.rbegin(),x.rend()
#define yes cout << "YES" << '\n';
#define no cout << "NO" << '\n';
#define mem(arrr, xx) memset(arrr,xx,sizeof arrr)
#define endl '\n'
#define PI acos(-1)
#define Ones(n) __builtin_popcount(n)
#define Onesl(n) __builtin_popcountll(n)
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
const ll MOD = 1e9 + 7;
const ll N = 1e6 + 5;
const int OO = 0x3f3f3f3f;
const ll LOO = 0x3f3f3f3f3f3f3f3f;
int dx8[] = {+0, +0, -1, +1, +1, +1, -1, -1};
int dy8[] = {-1, +1, +0, +0, +1, -1, +1, -1};
int dx4[] = {+0, +0, -1, +1};
int dy4[] = {-1, +1, +0, +0};

void Fast() {
    cin.tie(nullptr), cout.tie(nullptr), cin.sync_with_stdio(false), cout.sync_with_stdio(false);
#ifdef Clion
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}

vector<string> v;
int msk[N];
int get(string & s){
    int m = 0;
    for (int i = 0; i < s.size(); ++i) {
        m |= (1 << (s[i] - 'a'));
    }
    return m;
}
bool solve(vector<int> ind){
    if(ind.size() == 1){
        return true;
    }
    int mask = (1 << 26) - 1;
    for (int i = 0; i < ind.size(); ++i) {
        mask &= msk[ind[i]];
    }
    if(mask == 0)return false;
    map<string, vector<int>> mp;
    for (int i = 0; i < ind.size(); ++i) {
        string pat;
        for (int j = 0; j < v[ind[i]].size(); ++j) {
            if(mask & (1 << (v[ind[i]][j] - 'a'))){
                pat += v[ind[i]][j];
            }
            else{
                pat += '$';
            }
        }
        mp[pat].push_back(ind[i]);
    }
    // multiple strings
    if(mp.size() == 1){
        return false;
    }
    for(auto &f : mp){
        if(!solve(f.second)){
            return false;
        }
    }
    return true;
}
void Solution() {
    int l, n;
    cin >> l >> n;
    v.resize(n);
    vector<int> ind;
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
        ind.push_back(i);
        msk[i] = get(v[i]);
    }
    if(solve(ind)){
        cout << "YES\n";
    }
    else{
        cout << "NO\n";
    }
}

int main() {
    Fast();
    int T = 1;
//    cin >> T;
    while (T--) {
        Solution();
    }
}