#include <stdio.h>

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