#include <bits/stdc++.h>
using namespace std;
int main(){
    int n; cin >> n;
    int rel;
    for(int i=0; i < n; i++){
        int x,y; cin >> x >> y;
        if(i==0) rel = y - x;
        else if(rel != y - x){
            cout << -1 << endl;
            return 0;
        }
    }
    if(rel > 0){
        cout << rel << endl;
    } else{
        cout << -1 << endl;
    }
}