Hidden Number

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

ifstream in("hidden_number.in");
ofstream out("hidden_number.out");

int main()
{
    int n,ap=0,s=0;
    while(in>>n)
        s+=n,ap++;
    if(s%ap==0)
        out<<s/ap;
    else
        out<<-1;
	return 0;
}