#include <iostream>
#include <cstdint>
#include <cmath>
#include <string>
#include <vector>
#include <map>
#include <algorithm>

int main(void)
{
    int64_t R;

    std::cin >> R;

    std::cout << (double)(2*M_PI*R) << std::endl;

    return 0;
}