#include<iostream>
#include<algorithm>
#include<stdio.h>
#include<stdlib.h>
#include<vector>
#include<string>
#include<sstream>
#include<math.h>
using namespace std;


int main(){

  int n;
  cin >> n;
  vector<int> x(n);
  vector<int> y(n);

  for(int i=0;i<n;i++){
    cin >> x[i] >> y[i];
  }

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

  return 0;
}