#include<bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

#define ll long long
#define endl '\n'
using namespace std;
using namespace __gnu_pbds;

template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
const int N = 2e5+7, P1 = 31, P2 = 37, mod= 1e9 + 7;
int mul(int a, int b) {
    return (1LL * a * b) % mod;
}

int add(int a, int b) {
    a = (a + mod) % mod;
    b = (b + mod) % mod;
    return (a + b) % mod;
}

int fp(int b, int p) {
    if (b == 1 or p == 0)
        return 1;

    int ret = fp(b, p >> 1);
    ret = mul(ret, ret);

    if (p & 1)
        ret = mul(ret, b);

    return ret;
}
vector<string> v(N);
int pre[N][26];
int th[N][26];
bool solve(set<int> st,string s)
{
    for (char i = 'a'; i <= 'z'; ++i) {
        map<int,set<int>>mp;
        if ( std::count(s.begin(), s.end(), i))
            continue;
        int f =1;
        for(auto j : st)
        {
            if ( th[j][i-'a']) {
                mp[pre[j][i-'a']].insert(j);
            } else{
                f=0;
            }
        }
        if ( f == 0)
            continue;
        if ( mp.size() == st.size())
            return 1;
        else
        {
            int ans =1;
            for(auto it :mp )
            {
                if ( it.second.size() > 1)
                {
                    ans&= solve(it.second,s+i);
                }
            }
            if ( ans == 1)
                return ans;
        }
        break;
    }
    return 0;
}
int arr[26][N];
void doWork() {
    int l,n;
    cin >> l >> n;
    set<int> st;
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
        st.insert(i);
    }
    string s ="";
    for (int j = 0; j < n; ++j) {
        for (int i = 0; i < l; ++i)
            pre[j][v[j][i]-'a'] +=   mul(i,fp(P1, th[j][v[j][i]-'a']++)),pre[j][v[j][i]-'a']%=mod;
    }
    if ( solve(st,s))
        cout << "YES\n";
    else
        cout << "NO\n";
}



int main() {
    ios::sync_with_stdio(false);
    cout.tie(nullptr);
    cin.tie(nullptr);
//    freopen("bisector.in","r",stdin);
//    freopen("bisector.out","w",stdout);
    int t = 1;
    // cout << primes.size() << endl;
   // cin >> t;
    while (t--) {
        doWork();
    }
    return 0;
}