青岛市程序设计竞赛冲刺②

#include<bits/stdc++.h>
#define ll long long
using namespace std;
ll n,ans=0,t;
int main(){
	freopen("turn.in","r",stdin);
	freopen("turn.out","w",stdout);
	cin>>n;t=sqrt(n);
	if(n==2)cout<<0;
	else if(t*t!=n)cout<<-1;
	else{
		ans=n-2+1;
		while(t!=2){
			ll tmp=(t-1)*(t-1);
			ans+=tmp-t+1;
			t--;
		}
		cout<<ans;
	}
	return 0;
}