https://codeforces.com/contest/462/problem/A

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int n;
    cin >> n;
    vector<vector<bool>> vec(n + 2, vector<bool>(n + 2));
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++) {
            char ch; cin >> ch;
            vec[i][j] = 1 * (ch == 'o');
        }
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++) {
            if (int(vec[i - 1][j] + vec[i + 1][j] + vec[i][j - 1] + vec[i][j + 1]) % 2 != 0) {
                cout << "NO" << endl;
                return 0;
            }
        }
    cout << "YES" << endl;

    return 0;
}