#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<long long,long long> pll;
#define pb push_back
#define mp make_pair
#define rep(i,n) for(int i=0;i<(n);++i)
constexpr int mod=1000000007;
constexpr int mod1=998244353;
vector<int> dx={0,1,0,-1},dy={-1,0,1,0};
bool inside(int y,int x,int h,int w){
	if(y<h && y>=0 && x<w && x>=0) return true;
	return false;
}





int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int h,w;cin >> h >> w;
	vector<string> s(h);
	rep(i,h) cin >> s.at(i);
	bool ok = true;
	rep(i,h){
		bool black = false, white = false;
		rep(j,w){
			if(s[i][j] == '0') black = true;
			else white = true;
		}
		if(!black || !white) ok = false;
	}
	if(ok){
		cout << "YES" << endl;
		return 0;
	}
	ok = true;
	rep(i,w){
		bool black = false, white = false;
		rep(j,h){
			if(s[j][i] == '0') black = true;
			else white = true;
		}
		if(!black || !white) ok = false;
	}
	if(ok) cout << "YES" << endl;
	else cout << "NO" << endl;
}