#include <bits/stdc++.h>
using namespace std;
long long int gcd(long long int a, long long int b) {
  if (a < b) return gcd(b, a);
  long long int r;
  while (r = a % b) {
    a = b;
    b = r;
  }
  return b;
}
int main() {
  double r;
  cout << 2 * 3.1415 * r << endl;
  return 0;
}
