#include <iostream>
using namespace std;
int main() {
	int n, m;
	cin >> n >> m;
	int num[n][m] = {0}, ans = 0;
	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < m; ++j) {
			cin >> num[i][j];
		}
	}
	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < m; ++j) {
			if ((num[i][j] != num[i - 1][j]) && (num[i][j] != num[i + 1][j]) && (num[i][j] != num[i][j - 1]) && (num[i][j] != num[i][j + 1])) {
				ans++;
				for (int a = 0; a < m; ++a) {
					num[i][a] = num[i][a] xor 1;
				}
			}
		}
	}
	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < m; ++j) {
			if ((num[i][j] != num[i - 1][j]) && (num[i][j] != num[i + 1][j]) && (num[i][j] != num[i][j - 1]) && (num[i][j] != num[i][j + 1])) {
				cout << "-1";
				exit(0);
			}
		}
	}
	cout << ans;
}