#include <iostream>
#include <map>
using namespace std;

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