#include <iostream>
#include <algorithm>
#include <cstring>
#include <cmath>
#include < iomanip>
using namespace std;
int r;

#define pi 3.14159265358979323846

int main(){
    cin >> r;
    double rez = 2 * pi * r;
    cout << fixed << setprecision(20) << rez;
    return 0;
}