#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair

using namespace std;

const int INF = 2000000000;
const double pi = 3.1415926;
const int base = 1000000007;

double n;

int main()
{
//    ifstream cin ("house.in");
//    ofstream cout ("house.out");
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cout.precision(25);
    cout.setf(ios_base::fixed);
    cin >> n;
    cout << 2 * pi * n;
    return 0;
}
