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

int main(){
    int N,x,y,ans=0;

    cin >> N;
    while(cin >> x >> y){
        if(y-x<1 || (ans!=0&&ans!=y-x)){
            cout << -1 << endl;
            return 0;
        }else{
            ans=y-x;
        }
    }

    cout << ans << endl;

    return 0;
}