Problem 16 A FLAGS

#include<bits/stdc++.h>
using namespace std;
#define int long long
#define endl "\n"
const int mod=1e9+7;
#define fast_io ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define clr(a,x) memset(a,x,sizeof(a))
#define T int t;cin>>t; while(t--)
#define vll vector<int>
#define pb push_back
#define sz size()
int32_t main()
{
  int n,m;
  cin>>n>>m;
  char ch;
  int a[n][m],b=0;
  for(int i=0;i<n;i++)
  {
    for(int j=0;j<m;j++)
    {
     cin>>ch;
     a[i][j]=ch;
    }
  }
  for(int i=0;i<n;i++)
  {
    for(int j=0;j<m-1;j++)
    {
      if(a[i][j]!=a[i][j+1])
      {
        b=1;
        break;
      }
    }
    if(i!=(n-1))
    {
    if(a[i][0]==a[i+1][0])
    {
      b=1;
      break;
    }
    }
  }
  if(b==1)
  cout<<"NO";
  else 
  cout<<"YES";
}