#include <iostream>
#include <iomanip>

constexpr auto PI = 3.14159'26535'89793'23846'26433'83279'5028;

int main()
{
    int R;
    std::cin >> R;

    std::cout << std::setprecision(10) << 2 * R * PI << std::endl;

    return 0;
}