#include<iostream>
using namespace std;

int n;
int x, y;

int main() {
cin >> n >> x >> y;
int ans = y - x;
for (int i = 1; i < n; i++) {
cin >> x >> y;
if (ans != y - x || ans <= 0) {
cout << -1 << endl;
return 0;
}
}
cout << ans << endl;
return 0;
}