//  Copyright © 2020 Diego Garcia Rodriguez del Campo. All rights reserved.
#include<bits/stdc++.h>
#define MAX 200005
#define optimiza_io cin.tie(0); ios_base::sync_with_stdio(0);
#define forn(i,a,b) for (int i = a; i <= b; i ++ )
#define mod(n,k) (((n%k)+k)%k)
using namespace std;
typedef long long ll;

int main()
{
    optimiza_io
    long double R;
    cin >> R;
    cout << fixed << setprecision( 10 ) << R * 2 * 3.1415926535897932 << "\n";
    return 0;
}

// CHECAR LIMITES 