#include<bits/stdc++.h>
#define int long long
#define eps 1e-9
#define emailam  ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define endl '\n'
#define ll              long long
#define ull             unsigned long long
#define pb push_back
#define all(a)  a.begin(),a.end()
#define pf push_front
#define fi first
#define se second
#define pii pair<int,int>
const int INF = INT_MAX;
using namespace std;
const int N = 1e6 + 10;
/*----------------------------------------------------------------*/
int dx[] = {+0, +0, -1, +1, +1, +1, -1, -1};
int dy[] = {-1, +1, +0, +0, +1, -1, +1, -1};

/*----------------------------------------------------------------*/
void READ() {
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}

int n,m;
bool vis[N][26];
vector<string>v;
void get(vector<int>&indices,int msk){
    if(indices.size()<=1){
        return;
    }
    int x=INF; // char to take.
    for(int i=0;i<26;i++) {
        if((1<<i)&msk) {
            bool flag = true;
            for (auto it: indices) {
                if (vis[it][i])continue;
                flag = false;
                break;
            }
            if (flag) {
                x = i;
                break;
            }
        }
    }
    if(x==INF){
        cout<<"NO"<<endl;
        exit(0);
    }
    map<vector<int>,vector<int>>mp;
    for(auto i : indices){
        vector<int>k;
        for(int j=0;j<v[i].size();j++){
            if(v[i][j]-'a'==x){
                k.push_back(j);
            }
        }
        mp[k].push_back(i);
    }
    msk^=(1<<x);
    for(auto it : mp){
        get(it.second,msk);
    }
}
void solve() {
    cin>>m>>n;
    vector<int>indices;
    set<int>c;
    for(int i=0;i<n;i++){
        string s;
        cin>>s;
        v.push_back(s);
        for(auto it : s){
            vis[i][it-'a']=true;
        }
        indices.push_back(i);
    }
    get(indices,(1<<26)-1);
    cout<<"YES"<<endl;

}


signed main() {
    emailam
    //READ();
    int t = 1;
    //cin >> t;
    while (t--) {
        solve();
    }
}