#include <bits/stdc++.h>
using namespace std;
const long long INFLL = 1e18;
const int MAXN = 1e6 + 100;
const long long INF = 1e9;
const long long mod1 = 1e9 + 7;
const long long mod2 = 2e9 + 11;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  double pi = acos(-1);
  double r;
  cin >> r;
  cout << setprecision(30) << r * pi * 2;
}
