#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <string>
#include <iomanip>
#include <algorithm>
#include <vector>
#include<bits/stdc++.h>
using namespace std;

int N, M;
string S;
long double R;

int main(){

    cin >> R;

    cout << 2.0*acos(-1)*R;


}
