#include <bits/stdc++.h>
using namespace std;
const double PI = 3.14159265358979323846;
long long gcd(long long a, long long b) {
  if (a < b) return gcd(b, a);
  long long r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
int main() {
  int R;
  cin >> R;
  cout << 2 * R * PI << endl;
}
