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

int main() {
  int N;
  cin >> N;
  int ans;
  for (int i = 0; i < N; i++) {
    int x, y;
    cin >> x >> y;
    if (i && y - x != ans) {
      cout << -1 << endl;
      return 0;
    }
    ans = y - x;
  }
  cout << (ans > 0 ? ans : -1) << endl;
  return 0;
}