#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cassert>
#include <vector>
#include <string>
#include <cmath>
#include <map> 
#include <sstream>
#include <cstdio>
using namespace std;
 
const int MAX= 10000100;

#define loop(i,a,b) for(int i = a ; i < b ; i ++)
#define rep(i,a) loop(i,0,a)
#define all(a) (a).begin(),(a).end()
#define ll long long int
#define gcd(a,b) __gcd(a,b)
#define pb(a) push_back(a)
int GCD(int a, int b) {if(!b) return a; return gcd(b, a%b);}
int lcm(int a, int b) {return a*b / gcd(a, b);}

int main(void){
  int n;
  cin>>n;
  vector<ll> x(n);
  vector<ll> y(n);
  bool res = true;
  ll num;
  cin>>x[0]>>y[0];
  if(x[0] >= y[0])res = false;
  num = y[0] - x[0];
  loop(i,1,n){
    cin>>x[i]>>y[i];
    //    cout<<y[i] - x[i] << " == " << num<<endl; 
    if(x[i] >= y[i] || !res || y[i] - x[i] != num){
      res = false;
      break;
    }
  }

  if(res){
    cout<<num<<endl;
  }else{
    cout<<-1<<endl;
  }
}