#!/usr/bin/perl
use warnings;
use strict;

our $x = shift(@ARGV);
our $N = 80;

my $twopi = 8.0*atan2(1.0,1.0);
my $total = 0.5;
my $term  = $x/sqrt($twopi);

for my $k (0 .. $N) {
  if ($k > 0) {
    $term *= (-$x*$x)/(2.0*$k);
  }
  $total += $term/(2.0*$k+1.0);
}

print "$total\n";

