#include <stdio.h>

int main(void) {
	int i, N, a[1001], b[1001], c, max, min;
	scanf("%d", &N);
	max = 0;
	min = 10000000;
	for(i = 1;i <= N;i++){
		scanf("%d%d", &a[i], &b[i]);
		c = a[i] + 4 * b[i];
		if(c > max)max = c;
		if(c < min)min = c;
	}
	if((max - min) % 2 == 1){
		printf("-1\n");
		return 0;
	}
	//printf("%d %d %d %d %d\n", a[i], b[i], c[i], max, min);
	printf("%d\n", (max - min) / 2);

    return 0;
}