#include <bits/stdc++.h>
using namespace std::literals::string_literals;
using i64 = std::int_fast64_t;
using std::cout;
using std::endl;
using std::cin;

template<typename T>
std::vector<T> make_v(size_t a){return std::vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
  return std::vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

int main() {
	// input
	int h, w; scanf("%d%d", &h, &w);
	assert(1 <= h);
	assert(1 <= w);
	assert(h * w <= (int)1e5);
	
	std::vector<std::string> s(h);
	for(int i = 0; i < h; i++) {
		cin >> s[i];
		
		assert(s[i].size() == w);
		for(auto c: s[i]) assert(c == '1' or c == '0');
	}
	
	// solve
	std::vector<int> dx = {0, 1, 0, -1};
	std::vector<int> dy = {1, 0, -1, 0};
	for(int i = 0; i < h; i++) {
		for(int j = 0; j < w; j++) {
			int cnt = 0, cell = 0;
			for(int k = 0; k < 4; k++) {
				int nx = i + dx[k], ny = j + dy[k];
				if(nx < 0 or ny < 0 or nx >= h or ny >= w) continue;
				cnt += (s[i][j] == s[nx][ny]);
				cell++;
			}
			if(cnt == cell) {
				printf("NO\n");
				return 0;
			}
		}
	}
	
	printf("YES\n");
	return 0;
}