#include <bits/stdc++.h>
using namespace std;

int main() {
  int n, d, t, x, y;
  bool f = true;
  cin >> n;
  cin >> x >> y;
  d = y - x;
  if (d <= 0) f = false;
  for (int i = 1; i < n; i++) {
    cin >> x >> y;
    if (!f) continue;
    t = y - x;
    if (t <= 0 || d != t) f = false;
  }
  cout << (f ? d : -1) << endl;
  return 0;
}