#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#define REP(i, n) for (int i = 0; i < (n); i++)

using namespace std;

int main() {
  int H, W;
  cin >> H >> W;
  vector<string> S(H);
  REP(i, H) cin >> S[i];
  int a[2] = {};
  REP(i, H) REP(j, W) {
    if (S[i][j] == 'o') {
      a[(i + j) % 2]++;
    }
  }
  puts((a[0] - a[1]) % 3 == 0 ? "YES" : "NO");
}