$v[$i++] = $_ for 'a'..'t';

@a = qw/2 13 3 16/;
$a = join '', map {$v[$_]} @a;

#generating 1 000 000 base of numbers

for (0..1000000){
	%rnd = ();
	while(keys %rnd < 4){$rnd{$v[int(rand(20))]} = undef;}
	push @b, join '', keys %rnd;
}

#doing search

for (@b){
	$counter++ if !/[$a]/;
}

print "$counter\n";