#include <iostream>

using namespace std;

int h, w;

int main(void)
{
	cin >> h >> w;
	
	int cnt = 0;
	for(int i = 0; i < h; i++){
		for(int j = 0; j < w; j++){
			char c;
			cin >> c;
			if(c == 'o') cnt += max(h-1-i, j)%2+1;
		}
	}
	if(cnt%3 == 0) cout << "YES" << endl;
	else cout << "NO" << endl;
	return 0;
}