#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include<cstring>
using namespace std;
 
int main(){

  int n, x[1000], y[1000], ans;
  cin >> n;
  for(int i=0; i<n; i++){
    cin >> x[i] >> y[i];
    ans=y[0]-x[0];
    if(ans<0){
      cout << -1 << endl;
      return 0;
    }
  }

  for(int i=1; i<n; i++){
    if(ans!=(y[i]-x[i])){
      cout << -1 << endl;
      return 0;
    }
  }

  cout << ans << endl;

  return 0;
}