#include <bits/stdc++.h>
#include <vector>
#include <string>
#include <stdio.h>
#include <cstdlib>
#include <iostream>
#include <cstdlib>
using namespace std;
using ll = long long;
#define mod 1000000007

int main()
{
    float r, circum;
	cin>>r;
	circum=2*3.141592653589793238462643383*r;
	printf("%.7f\n",circum);
    return 0;
}
