#include <iostream>
#include <stdio.h>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <math.h>

using namespace std;

int main(){
    double r, ans;
    cin >> r;
    ans = 2.0*r*M_PI;
    cout << fixed << ans;
    return 0;
}
