// includes {{{
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<tuple>
#include<cmath>
#include<random>
#include<cassert>
#include<bitset>
#include<cstdlib>
// #include<deque>
// #include<multiset>
// #include<cstring>
// #include<bits/stdc++.h>
// }}}
using namespace std;
using ll = long long;

int HW_MAX = 100'000;

int main() {
  std::ios::sync_with_stdio(false), std::cin.tie(0);
  int h, w;
  cin >> h >> w;

  assert(1 <= h and h <= HW_MAX);
  assert(1 <= w and w <= HW_MAX);
  assert(2 <= h * w and h * w <= HW_MAX);

  vector<string> f(h);
  for(int i = 0; i < h; i++) {
    cin >> f[i];
    assert(int(f[i].size()) == w);
    for(auto e : f[i]) assert(e == '0' || e == '1');
  }

  int eca = 0;
  for(int i = 0; i < h; i++) {
    bool same = 1;
    for(int j = 0; j < w; j++) same &= f[i][j] == f[i][0];
    if(same) eca |= 0b01;
  }
  for(int j = 0; j < w; j++) {
    bool same = 1;
    for(int i = 0; i < h; i++) same &= f[i][j] == f[0][j];
    if(same) eca |= 0b10;
  }

  if(eca == 0b11) {
    cout << "NO" << endl;
  } else {
    cout << "YES" << endl;
  }
  return 0;
}