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

int main() {
    int n, a, b,c, f=0;
    cin >> n;
    n--;
    cin >> a>>b;
    c = b-a;
    if (c <= 0) f=1;
    while (n--) {
        cin>>a>>b;
        if (b-a != c){
            cout<<-1<<endl;
            return 0;
        }
    }
    if(f==1) cout<<-1<<endl;
    else cout << c <<endl;
    return 0;
}