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sub permutation {
    my ($prob, $start) = (1, shift);
    $prob *= $start-- foreach (1 .. $_[0]);
    $prob;
}

sub permutation {
    my $prob = 1;
    $prob *= $_ foreach ((($_[0]+1) - $_[1]) .. $_[0]);
    return $prob;
}

sub permutation {
    my $prob = 1;
    return (map { $prob *= $_ } (($_[0] - $_[1] + 1) .. $_[0]))[-1];
}