#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

const int M = 1'000'000;
void solve() {
  ll n;
  cin >> n;
  double ans = 0;
  rep(i, M) {
    double x = (double(i) + 0.5) / double(M);
    double y = sqrt(double(x * x * x + n * n * n));
    ans += y / double(M);
  }
  cout << fixed << setprecision(6) << ans << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}