read_string(S) :-
    read_token_codes(S).

read_int(I) :-
    read_token_codes(Codes),
    (Codes == end_of_file -> I = Codes ; number_codes(I, Codes)).

read_atom(A) :-
    read_token_codes(Codes),
    (Codes == end_of_file -> A = Codes ; atom_codes(A, Codes)).

read_token_codes(end_of_file) :-
    peek_code(end_of_file), !.

read_token_codes(Codes) :-
    peek_code(C),
    \+ code_type(C, space), !,
    read_token_codes_helper(Codes).

read_token_codes(T) :-
    get_char(_), !,
    read_token_codes(T).


read_token_codes_helper([C0|C]) :-
    peek_code(C0),
    \+ code_type(C0, space), !,
    get_code(C0),
    read_token_codes_helper(C).

read_token_codes_helper([]).

main :-
    read_int(N),
    read_int(P),
    M is mod(N, P),
    Q is div(N, P),
    (   M == 0
    ->  write(0)
    ;   Res is N - (Q - mod((Q + 1), 2)) * P,
        write(Res)
    ),
    nl.