#include <stdio.h>

int main(void)
{
	long n;
	long x[1000], y[1000];
	long diff;
	int i;

	scanf("%ld", &n);

	for (i = 0; i < n; i++) 
		scanf("%ld %ld", &x[i], &y[i]);

	diff = y[0] - x[0];
	for (i = 1; i < n; i++) {
		if (y[i] - x[i] != diff) {
			diff = -1;
			break;
		}
	}
	if (diff <= 0)
		diff = -1;
	printf("%ld\n", diff);

	return 0;
}