#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define ull unsigned ll

#define vi vector <int>
#define ii pair <int, int>
#define vii vector <ii>
#define vll vector <ll>
#define vull vector <ull>
#define INFL 9e18
#define INF 2e9
#define EPS 1e-9
#define pb push_back
#define emp emplace_back
#define PI acos(-1.0)
#define endl "\n"
#define mp make_pair
#define DEBUG 0

int main ()

{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    double r;
    cin >> r;
    
    printf ("%.6lf\n", 2*PI*r);
    
    return 0;
}