#include <bits/stdc++.h>   using namespace std;   bool IO_FILE = 0;   ifstream ifs( input.txt );   ofstream ofs( output.txt );   int main() {     if (IO_FILE) {       cin.rdbuf(ifs.rdbuf());       cout.rdbuf(ofs.rdbuf());     }     int n, m;     cin >> n >> m;     char *s = new char[m];     bool res = 1;     char prev =  E ;     for (int i = 0; i < n; i++) {       cin >> s;       char p = s[0];       res = 1;       if (prev !=  E  && p == prev) {         res = 0;         break;       }       for (int j = 1; j < m; j++)         if (!(res = s[j] == p)) break;       if (!res) break;       prev = p;     }     cout << (res ?  YES  :  NO );   }   