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

#define el '\n'
#define FIO ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef long double ld;
typedef complex<ld> pt;
typedef unsigned long long ull;

template<typename T, typename X>
using hashTable = gp_hash_table<T, X>;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

// mt19937_64 for long long
mt19937 rng(std::chrono::system_clock::now().time_since_epoch().count());

const int N = 1e5 + 5;

vector<string> v;
vector<int> idx[N][26];

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


    for(int c = 0; c < 26; c++)
    {
        bool valid = true;
        map<vector<int>, vector<int>> mp;
        for(auto &it: cur)
        {
            if(idx[it][c].empty())
            {
                valid = false;
                continue;
            }

            mp[idx[it][c]].push_back(it);
        }

        if(!valid || mp.size() == 1) // el 7rf dah m4 mawgod f kolo
            continue;

        bool ret = true;
        for(auto &it: mp)
            ret &= solve(it.second);
        return ret;
    }
    return false;
}

void doWork()
{
    int n, m;
    cin >> m >> n;
    for(int i = 0; i < n; i++)
    {
        v.emplace_back(), cin >> v.back();
        for(int j = 0; j < m; j++)
            idx[i][v.back()[j] - 'a'].push_back(j);
    }

    vector<int> cur(n);
    iota(cur.begin(), cur.end(), 0);
    cout << (solve(cur) ? "YES" : "NO");
}

signed main()
{
    FIO
    int T = 1;
//    cin >> T;
    for(int i = 1; i <= T; i++)
        doWork();
}