#include <bits/stdc++.h>
using namespace std;
const int MODU = 1e9 + 7;
const int N = 1e5 + 10;
void solve() {}
int main() {
  const double PI = 2 * 3.1415926;
  cout << setprecision(12);
  int r;
  cin >> r;
  cout << PI * r << endl;
  return 0;
}
