#include <stdio.h>

int main() {
	int n, a[1000], b[1000], i;
	long long int max, sum = 0;

	scanf("%d", &n);
	scanf("%d %d", &a[0], &b[0]);
	max = a[0] + b[0] * 4;
	for (i = 1; i < n; i++) {
		scanf("%d %d", &a[i], &b[i]);
		if ((a[i] + b[i] * 4) % 2 != 
			(a[i - 1] + b[i - 1] * 4) % 2) {
			printf("-1\n");
			return 0;
		}
		if (max < a[i] + b[i] * 4)
			max = a[i] + b[i] * 4;
	}

	for (i = 0; i < n; i++) {
		sum += (max - (a[i] + b[i] * 4)) / 2;
	}

	printf("%lld\n", sum);

	return 0;
}