#include <stdio.h>

int main(void)
{
	int n, x, y, i = 0, t = 0;
	scanf("%d", &n);
	for (i; i < n; i++) {
		scanf("%d%d", &x, &y);
		if (i > 0 && t != y - x || x >= y) {
			t = -1;
			break;
		}
		t = y - x;
	}
	printf("%d\n", t);
	return 0;
}