#include <stdio.h>

int main(void) {
	int i, N;
	long long int a[1001], b[1001], c[1001], max, sum;
	scanf("%d", &N);
	max = 0;
	for(i = 1;i <= N;i++){
		scanf("%d%d", &a[i], &b[i]);
		c[i] = a[i] + 4 * b[i];
		if(c[i] > max)max = c[i];
	}
	sum = 0;
	for(i = 1;i <= N;i++){
		if((max - c[i]) % 2 == 1){
			printf("-1\n");
			return 0;
		}
		sum += max - c[i];
	}
	printf("%lld\n", sum / 2);

    return 0;
}