#include <iostream>
#include <list>
#include <vector>
#include <string>
#include <stdio.h>
#include <stdint.h>
using namespace std;

using uint = unsigned int;
using ll = long long;

#define CIN( LL , A ) LL A; cin >> A 
#define GETLINE( A ) string A; getline( cin , A ) 

int main()
{

  CIN( ll , N );
  ll answer = -1;
  ll X;
  ll Y;

  for( ll i = 0 ; i < N ; i++ ){

    cin >> X;
    cin >> Y;

    const ll d = Y - X;

    if( d <= 0 ){

      answer = -1;
      i = N;

    } else if( answer == -1 ){

      answer = d;

    } else if( answer != d ){

      answer = -1;
      i = N;

    }

  }

  cout << answer << endl;
  return 0;

}