//Normal

#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

void printl(auto a){
  cout << a << endl;
}

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int main(){
  int N; cin >> N;
  int ans = 1e9;
  
  for(int i = 0; i < N; i++){
    int X, Y; cin >> X >> Y;
    int Z = Y - X;
    
    if(ans != Z){
      if(ans == 1e9){
        ans = Z;
      }else{
        printl(-1);
        return 0;
      }
    }
  }
  
  ans >= 1 ? printl(ans) : printl(-1);
  
  return 0;
}