#include <bits/stdc++.h>
using namespace std;
int main() {
  int x;
  cin >> x;
  float s = 1;
  float pi = 0;
  float i = 1.0;
  float n = 1.0;
  while (fabs(i) >= 1e-6) {
    pi += i;
    n = n + 2;
    s = -s;
    i = s / n;
  }
  pi = 4 * pi * x;
  printf("%.30f\n", pi);
  return 0;
}
