#include<iostream>

template<typename T, int size>
struct PreCirclePond
{
    T ary[size];
    static constexpr float pi = 3.141592;

    constexpr PreCirclePond() : ary() {
        for (int i = 0; i < size; i++)
        {
            const int t = i * 2;
            ary[i] = t * pi;
        }
    }

    T take(int r) const {
        return ary[r];
    }
};

template<class R>
float ciclePondP(R r) {
    constexpr auto a = PreCirclePond<float, 100>();
    return a.take(r);
}

int main() {
    using namespace std;
    ios_base::sync_with_stdio(false);

    int R;
    cin >> R;

    cout << ciclePondP(R);
}