#include <stdio.h>
#include <algorithm>
#include <deque>
using namespace std;
using ll = long long;

//185
int main() {
	int n, x, y, diff, imp = 0;
	scanf("%d %d %d", &n, &x, &y);
	diff = y - x;
	for (int i = 2; i <= n; i++) {
		scanf("%d %d", &x, &y);
		if (diff + x != y)
			imp = 1;
	}
	if (imp)
		printf("-1\n");
	else
		printf("%d\n", diff);
}