#include <stdio.h>

int main(void)
{
	int n, a, b, i;
	long s[100] = { 0 }, m = 0, ans = 0;
	scanf("%d", &n);
	for (i = 0; i < n; i++) {
		scanf("%d%d", &a, &b);
		s[i] += a + 4 * b;
		if (m < s[i]) m = s[i];
	}
	for (i = 0; i < n; i++) ans += m - s[i];
	printf(ans % 2 ? "-1" : "%d", ans / 2);
	return 0;
}