#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <iostream>
#include <utility>
#include <vector>
#include <list>
#include <algorithm>
using namespace std;

#define ll long long

int main()
{
    ll N;
    scanf("%lld", &N);

    printf("%lf\n", N * 2 * 3.141592);
}
