#include <bits/stdc++.h>

using namespace std;

int main(void) {
  int n;
  cin >> n;

  int ans = -1;
  bool no = false;
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    no |= (y - x < 0 || (ans > 0 && ans != y - x));
    ans = y - x;
  }

  cout << (ans > 0 && !no ? ans : -1) << endl;

  return 0;
}