loop() ->
    receive
        {From, Msg} ->
            if
                is_integer(Msg), Msg >= 0 ->
                    From ! {self(), factorial(Msg)},
                    loop();
                Msg == stop ->
                    From ! {self(), papa};
                true ->
                    From ! {self(), Msg},
                    loop()
            end
    end.

start() ->
    spawn(echo, loop, []).

send(Pid, MSG) ->
    B = is_process_alive(Pid),
    if
        B ->
            Pid ! {self(), MSG},
            receive
                {Pid, Msg} ->
                    io:format("~w~n", [Msg])
                end;
        true ->
            io:format("Watek nie zyje~n")
    end.

factorial(N) ->
    if
        N == 0 -> 1;
        N > 0 -> N * factorial(N-1)
    end.