#include <bits/stdc++.h>
using namespace std;

signed main(){
    int n;cin>>n;
    set<int> s;
    while(n--){
        int a,b;cin>>a>>b;
        s.insert(b-a);
    }
    if(s.size()>1||*s.begin()<=0)cout<<-1<<endl;
    else cout<<*s.begin()<<endl;
}