#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long ul;
typedef signed long long ll;

int main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed;
  int n;
  cin >> n;
  ll x, y;
  cin >> x >> y;
  ll ans = y-x;
  if (ans <= 0) {cout << -1 << endl; return 0;}
  for (int i = 1; i < n; ++i) {
    cin >> x >> y;
    if (ans != y-x) {cout << -1 << endl; return 0;}
  }
  cout << ans << endl;
  return 0;
}