#include <bits/stdc++.h>
 
using namespace std;
  
int main() {
  int N, X, Y;
  cin >> N >> X >> Y;
  int x = Y - X;
  if (x <= 0) {
    cout << -1 << endl;
    return 0;
  }
  for (int i = 1; i < N; i++) {
    cin >> X >> Y;
    if (x != Y - X) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << x << endl;
  return 0;
}