#include <stdio.h>
#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define modulo 1000000007
#define mod(mod_x) ((((long long)mod_x+modulo))%modulo)
#define Inf 10000000



int main(){
	
	int h,w;
	cin>>h>>w;
	
	vector<string> S(h);
	
	rep(i,h)cin>>S[i];
	int sum = 0;
	rep(i,h){
		rep(j,w){
			if(S[i][j]=='o'){
				if((j+(h-1-i))%2==0)sum++;
				else sum += 2;
			}
		}
	}
	
	if(sum%3==0)cout<<"YES"<<endl;
	else cout<<"NO"<<endl;
	
	
    return 0;
}