#include <bits/stdc++.h>

using namespace std;

int main()
{
  int N, X[1000], Y[1000];

  cin >> N;
  for(int i = 0; i < N; i++) {
    cin >> X[i] >> Y[i];
  }

  int ret = Y[0] - X[0];
  for(int i = 0; i < N; i++) {
    if(Y[i] - X[i] != ret || ret <= 0) {
      cout << -1 << endl;
    }
  }
  cout << ret << endl;
}