#include <bits/stdc++.h>
#include <stdio.h>
#include <algorithm>
#define rep(i,n) for(int i=0; i<n; ++i)
using name space std;

int main(){
    double pi = 3.1415;
    double r;
    cin >> r;
    
    double ans;
    ans = 2 * pi * r;

    cout << ans << endl;
};