#include <bits/stdc++.h>
using namespace std;
using LL = long long;
constexpr int N = 1e5 + 5;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int r;
  cin >> r;
  const double pi = acos(-1.0);
  cout << 2 * pi * r << endl;
}
