#include <bits/stdc++.h>
using namespace std;
using ll = long long;
string char_to_string(char val) { return string(1, val); }
int char_to_int(char val) { return val - '0'; }
int main() {
  int R;
  cin >> R;
  cout << std::fixed << std::setprecision(15) << 2 * R * 3.1415926535 << endl;
}
