#define _USE_MATH_DEFINES
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>

typedef long long ll;

using namespace std;



int main() {
    double r;
    cin >> r;

    cout << r * 2 * M_PI;
   

    return 0;
}