//-- Araz Jabrayilov --
//-- araz17 - zuna#1041 --

#include <bits/stdc++.h>

#define ll long long
#define endl '\n'

using namespace std;

int main()
{
    ios_base :: sync_with_stdio(0);
    cin.tie(0);

    int n = 0;

    cin >> n;

    cout << 2 * 3.14159265359 * n << endl;

    return 0;
}

