#include <stdio.h>
#include <stdlib.h>

int main()
{
	int i;
	int a, b;
	int max;
	int fullwidths = 0;
	int n;
	int sum[1001];	//sum[0]は使わない
	
	scanf("%d", &n);
	for (i = 1; i <= n; i++) {
		scanf("%d %d", &a, &b);
		sum[i] = a + 4 * b;
	}
	
	max = sum[1];
	for (i = 2; i <= n; i++) {
		if (abs(sum[i - 1] - sum[i]) % 2 == 1) {
			printf("-1\n");
			return 0;
		}
		else if (sum[i] > max) {
			fullwidths += (sum[i] - max) / 2 * (i - 1);
			max = sum[i];
		}
		else {
			fullwidths += (max - sum[i]) / 2;
		}
	}
	
	printf("%d\n", fullwidths);
	
	return 0;
}