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