#include <iostream>
#include <vector>
#define llint long long
#define inf 1e18

using namespace std;

llint h, w;
char c[100005];

int main(void)
{
  ios::sync_with_stdio(0);
  cin.tie(0);

  cin >> h >> w;
  for(int y = 0; y < h; y++){
    for(int x = 0; x < w; x++){
      cin >> c[y*w+x];
    }
  }
  bool row = false, col = false;
  for(int x = 0; x < w; x++){
    bool same = true;
    for(int y = 0; y < h; y++){
      if(c[y*w+x] != c[0*w+x]) same = false;
    }
    if(same) col = true;
  }
  for(int y = 0; y < h; y++){
    bool same = true;
    for(int x = 0; x < w; x++){
      if(c[y*w+x] != c[y*w+0]) same = false;
    }
    if(same) row = true;
  }
  if(row && col) cout << "NO" << endl;
  else cout << "YES" << endl;

  return 0;
}