#include <bits/stdc++.h>
#define fastIO (cin.tie(0), cout.tie(0), ios::sync_with_stdio(false))
using namespace std;

int main() {
  fastIO;

  int n;
  cin >> n;

  int cur = 0;
  for (int i = 0; i < n; ++i) {
    int x, y;
    cin >> x >> y;

    if (y - x <= 0) {
      cout << -1 << '\n';
      return 0;
    } else {
      if (i == 0)
        cur = y - x;
      else {
        if (cur != y - x) {
          cout << -1 << '\n';
          return 0;
        }
      }
    }
  }

  if (cur > 0)
    cout << cur << '\n';
  else
    cout << -1 << '\n';
  return 0;
}