#include <iostream>

using namespace std;

int h, w;
char c[205][205];

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