#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    int H, W; cin >> H >> W;
    vector<string> fi(H);
    for (int i = 0; i < H; ++i) cin >> fi[i];
    reverse(fi.begin(), fi.end());

    int a = 0, b = 0;
    for (int i = 0; i < H; ++i) {
        for (int j = 0; j < W; ++j) {
            if (fi[i][j] == 'o') {
                if ( (i + j) % 2 == 0 ) ++a;
                else ++b;
            }
        }
    }
    if ( (a - b) % 3 == 0 ) cout << "YES" << endl;
    else cout << "NO" << endl;
}