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

signed main(){
  int N; cin >> N;
  int ans = -1;
  for( int i = 0; i < N; ++i ){
    int x, y; cin >> x >> y;
    int d = y - x;
    if( d <= 0 )
      cout << -1 << endl,
      exit( 0 );
    if( ans == -1 )
      ans = d;
    else if( ans != d )
      cout << -1 << endl,
      exit( 0 );
  }
  cout << ans << endl;
  return 0;
}