#include <stdio.h>

int main(void)
{
	long n, a, b, i, s[1000] = { 0 }, m = 0, t = 0, ans = 0;
	scanf("%ld", &n);
	for (i = 0; i < n; i++) {
		scanf("%ld%ld", &a, &b);
		s[i] += a + 4 * b;
		if (m < s[i]) m = s[i];
	}
	for (i = 0; i < n; i++) {
		if ((m - s[i]) % 2) t++;
		ans += m - s[i];

	}
	printf(t ? "-1" : "%ld", ans / 2);
	return 0;
}