#pragma region Region_1

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define rrng(a) a.rbegin(), a.rend()
using ll = long long;
using P = pair<int, int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VP = vector<P>;
#define MOD 1000000007
const int INF = 1e9 + 10;

template <class T, class C>
void chmax(T& a, C b) {
    if (a <= b) a = b;
}
template <class T, class C>
void chmin(T& a, C b) {
    if (a >= b) a = b;
}

// const int dx[]{0, 1, 0, -1, -1, -1, 1, 1}, dy[]{1, 0, -1, 0, -1, 1, -1, 1};
// const int dx[]{0, 1, 0, -1}, dy[]{1, 0, -1, 0};

template <class T>
T sum(const vector<T>& v) {
    T res = 0;
    for (size_t i = 0; i < v.size(); ++i) res += v[i];
    return res;
}

/////////////////////////////////////////////////////////
// print like python
// https://qiita.com/Lily0727K/items/06cb1d6da8a436369eed
/////////////////////////////////////////////////////////
void print() { cout << endl; }

template <class Head, class... Tail>
void print(Head&& head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail) != 0) cout << " ";
    print(forward<Tail>(tail)...);
}

template <class T>
void print(vector<T>& vec) {
    for (auto& a : vec) {
        cout << a;
        if (&a != &vec.back()) cout << " ";
    }
    cout << endl;
}

template <class T>
void print(vector<vector<T>>& df) {
    for (auto& vec : df) {
        print(vec);
    }
}

#pragma endregion Region_1
/////////////////////////////////////////////////////////

int main() {
    //入力の高速化用のコード
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    std::cout << std::setprecision(15);
    //////////////////////////////////////////

    ll k;
    cin >> k;
    double ans = 0;
    if (k > 0) {
        srep(i, 1, 100000000) {
            ans += (double)1 / (double)i - (double)1 / (double)(i + k);
        }
        ans /= k;

    } else {
        double pi = M_PI;
        ans = powl(pi, 2) / 6;
    }
    print(ans);

    return 0;
}