#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; }