#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long lcm(long long c, long long d) { return c / gcd(c, d) * d; }
int main() {
  double r;
  cin >> r;
  cout << 2.0 * r * 3.14 << endl;
  return 0;
}
