#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,m,n) for(int i=m; i<n; ++i)
#define repl(i,m,n) for(ll i=m; i<n; ++i)

int main(){
    double N;
    cin >> N;
    double ans = (15.0 + 5.0*sqrt(5.0))*N*N*N / 12.0;
    printf("%.20f\n", ans);
    return 0;
}