#include <bits/stdc++.h>
using namespace std;
using ll = long long;
void _cin() {}
template <class Head, class... Tail>
void _cin(Head&& head, Tail&&... tail) {
  cin >> head;
  _cin(forward<Tail>(tail)...);
}
void _cout() { cout << "\n"; }
template <class Head, class... Tail>
void _cout(Head&& head, Tail&&... tail) {
  cout << head;
  _cout(forward<Tail>(tail)...);
}
int gcd(int a, int b) { return (b == 0) ? a : gcd(b, a % b); }
template <typename S, typename T>
ostream& operator<<(ostream& os, const pair<S, T>& p) {
  cout << "[" << p.first << ", " << p.second << "]";
  return os;
}
int main(void) {
  int a, b, c, d;
  _cin(a, b, c, d);
  printf("%.15f\n", a * 6.28318530717958623200);
}
