#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
const int inf = 1e9;
const int N = 2e5;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long double pi = 3.14;
  long double r;
  cin >> r;
  cout << 2 * pi * r << "\n";
  return 0;
}
