#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 1000000000000000000LL

int main(){
	
	int h,w;
	cin>>h>>w;
	vector<string> s(h);
	rep(i,h)cin>>s[i];
	bool f = true;
	rep(i,h){
		set<char> S;
		rep(j,w)S.insert(s[i][j]);
		if(S.size()==1)f = false;
	}
	if(f)cout<<"YES"<<endl;
	else{
		f =true;
		rep(i,w){
			set<char> S;
			rep(j,h)S.insert(s[j][i]);
			if(S.size()==1)f = false;
		}
	}
	if(f)cout<<"YES"<<endl;
	else cout<<"NO"<<endl;
	
	return 0;
}