#include<stdio.h>
int main()
{
	int n;
	scanf("%d", &n);
	int i;
	int a[1003], b[1003];
	for (i = 0; i < n; i++)
		scanf("%d %d", &a[i], &b[i]);
	int max = 0;
	for (i = 0; i < n; i++)
		if (max < a[i] + 4 * b[i])
			max = a[i] + 4 * b[i];
	int ans = 0;
	for (i = 0; i < n; i++)
	{
		if ((max - a[i] - 4 * b[i]) % 2 > 0)
		{
			printf("-1\n");
			return 0;
		}
		ans += max - a[i] - 4 * b[i];
	}
	printf("%d\n", ans / 2);
	return 0;
}