#include <bits/stdc++.h>
using namespace std;

int main(){
    int H, W;
    cin >> H >> W;
    int num[2] = {0};
    for(int i=H-1; i>=0; i--){
        string S;
        cin >> S;
        for(int j=0; j<W; j++) if(S[j] == 'o') num[(i+j)%2]++;
    }
    int sum = num[0] + num[1];
    bool win;
    if(sum % 2 == 0){
        int d = abs(num[0] - sum/2);
        win = (d%3 == 0);
    }else{
        int d = min(abs(num[0] - sum/2), abs(num[0] - sum/2 - 1));
        win = (d%3 == 1);
    }

    cout << (win ? "YES" : "NO") << endl;
 
    return 0;
}