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