my ($pocky, $bite) = split (/ /, <>);
chomp ($pocky, $bite);

my $loss = $bite * 2;#2人が同時にポッキーを食べるときになくなる長さ

if($pocky<=$bite){
    print "0\n";
}
elsif($pocky % $loss == 0){
    my $eat = ($pocky / $loss) - 1;
    $eat *= $bite;
    print "$eat\n";
}else{
    my $remainder = $pocky % $loss;
    my $eat = ($pocky - $remainder) / $loss;
    $eat *= $bite;
    print "$eat\n";
}