//https://codeforces.com/contest/1335/problem/

#include <bits/stdc++.h>
#define ll long long int
#define pll pair<ll, ll>
#define pii pair<int, int>

using namespace std;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    double tpi = 6.28318530717958623200;
    double x;
    cin >> x;
    double z = tpi * x;
    cout << setprecision(3) << fixed;
    cout << tpi << '\n';
    return 0;
}