#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
 
using namespace std;
 
#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(c) (c).begin(), (c).end()
#define rep(i,a,b) for(long long i=(a);i<(b);++i)
#define clr(a, b) memset((a), (b) ,sizeof(a))
 
#define MOD 1000000007

int main(){
  int n;
  cin>>n;
  set<int> se; 
  rep(i,0,n){
    int x,y;
    cin>>x>>y;
    se.insert(y-x);
  }
  if(se.sz>1){
    cout << -1 << endl;
  }
  else{
    set<int>::iterator itr = se.begin();
    cout << *itr << endl;
  }
  return 0;
}