valid permutations

program permutations;

function isConsonant(ch: Char): Boolean;
begin
    ch := UpCase(ch);
    isConsonant := not (ch in ['A', 'E', 'I', 'O', 'U']);
end;

function isValidChain(chain: string): Boolean;
begin
    isValidChain := isConsonant(chain[1]) and
                    (chain[2] in ['C', 'D', 'F']) and
                    ((UpCase(chain[3]) in ['A', 'E', 'I', 'O', 'U']) or (isConsonant(chain[3]) and (chain[3] <> chain[2]))) ;
end;

procedure generatePermutations(prefix, remaining: string; var counter: Integer);
var
    i: Integer;
begin
    if length(prefix) = 3 then
    begin
        if isValidChain(prefix) then
            counter := counter + 1;
        Exit;
    end;

    for i := 1 to length(remaining) do
        generatePermutations(prefix + remaining[i], Copy(remaining, 1, i - 1) + Copy(remaining, i + 1, length(remaining)), counter);
end;

var
    counter: Integer;
    str: string;
begin
    str := 'ABCDEF';
    counter := 0;
    generatePermutations('', str, counter);
    WriteLn('Количество подходящих цепочек: ', counter);
end.