#include <bits/stdc++.h>
#include <atcoder/all>
#define ll long long
using namespace std;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
    ll N,X,Y,Z;
    cin>>N>>X>>Y;
    if(Y<=X){cout<<-1<<endl; return 0;}
    Z=Y-X;
    N--;
    while(N--){
        cin>>X>>Y;
        if(Y-X!=Z){cout<<-1<<endl; return 0;}
    }

    cout<<Z<<endl;
}