#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
constexpr int MOD = 998244353;
#define ov4(a, b, c, d, name, ...) name
#define rep3(i, a, b, c) for(ll i = (a); i < (b); i += (c))
#define rep2(i, a, b) rep3(i, a, b, 1)
#define rep1(i, n) rep2(i, 0, n)
#define rep0(n) rep1(aaaaa, n)
#define rep(...) ov4(__VA_ARGS__, rep3, rep2, rep1, rep0)(__VA_ARGS__)
#define per(i, a, b) for(ll i = (a)-1; i >= (b); i--)
#define all(v) v.begin(), v.end()
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  ll N;
  cin>>N;
  double ans=0;
  const int K=100000;
  auto f=[N](double x){
    return sqrt(x*x*x+N*N*N);
  };
  rep(i,K){
    ans+=f((double(i)+0.5)/K)/K;
  }
  cout<<setprecision(10)<<fixed;
  cout<<ans<<'\n';
}