#include <stdio.h>
#include <iostream>
#include <vector>
#include <string>
#include <list>
#include <algorithm>
#include <set>
#include <queue>

using namespace std;
#define ll long long

int main()
{
    int t=1;
    //cin>>t;
    while (t--)
    {
        double n;
        scanf("%lf", &n);
        printf("%.6lf\n", 2*3.1416*n);
    }
    
    return 0;
}
/*

riaadmorshed@gmail.com

*/
/*
4
7 5 3
6 1 1
6 6 1
5 2 2
*/
