#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <fstream>
#include <numeric>

using namespace std;

#define max(a,b) a > b ? a : b
#define min(a,b) a < b ? a : b

#define X_MAX 100000
#define Y_MAX 100000
#define A_MAX 100000
#define B_MAX 100000
#define C_MAX 100000
//#define n_MAX 1000000000
#define M_PI 3.1415926535

int main()
{
    double R;

    cin >> R;

    cout << 2 * M_PI * R << endl;

    return 0;
}