#include<iostream>
#include<climits>
#include<cstdlib>
#include<algorithm>
#include<vector>
using namespace std;
int main(){
    int n;
    int ans;
    cin >> n;
    int x,y;
    cin >> x >>y;
    ans = y-x;
    for(int i=0;i<n-1;i++){
        cin >> x >> y;
        if(y-x != ans){
            ans=-1;
            break;
        }
    }
    if(ans<-1||ans==0)ans=-1;
    cout << ans <<endl;
}