#include <bits/stdc++.h>
using namespace std;
long long GCD(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return GCD(b, a % b);
}
long long LCM(long long a, long long b) { return a * b / GCD(a, b); }
unsigned GetDigit(unsigned num) { return std::to_string(num).length(); }
int tow(int n) {
  if (n == 0) return 1;
  int x = tow(n / 2);
  x *= x;
  if (n % 2 == 1) x *= 2;
  return x;
}
int KETA(int n) {
  int sum = 0;
  while (n > 0) {
    sum += n % 10;
    n /= 10;
  }
  return sum;
}
unsigned ND(unsigned num) { return std::to_string(num).length(); }
bool KIBN(string s) {
  for (int i = 0; i < s.size() / 2; i++) {
    if (s[i] != s[s.size() - 1 - i]) return false;
  }
  return true;
}
int main(void) {
  double r;
  cin >> r;
  printf("%.15f\n", r * 2 * 3.1415926535897932384626);
  return 0;
}
