sub isValid {
    my @s = split("", shift);
    
    my %h;
    my %c;

    foreach (@s) {
        $h{$_} +=1;
    }
    foreach (values(%h)) {
        $c{$_} +=1;
    }

    return "YES" if (keys(%c) == 1);    
    
    if (keys %c == 2) {
        my @keys = keys(%c);
        my ($freq1, $freq2) = @keys;
        my ($count1, $count2) = @c{@keys};

        if (($freq1 == 1 && $count1 == 1) || ($freq2 == 1 && $count2 == 1)) {
            return "YES";
        }

        if ((abs($freq1 - $freq2) == 1) && ($count1 == 1 || $count2 == 1)) {
            return "YES";
        }
    }

    return "NO";
}