#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

int main()
{
    int n;
    cin >> n;
    lb ans = 0;
    for(int i=0;i<=1000000;i++) {
        lb x = (i)*(lb)1/1000000;
        ans += sqrt(x*x*x+n*n*n)*(lb)1/1000000;
    }
    cout << fixed << setprecision(16) << ans <<endl;
    return 0;
}