-module(fourth).
-export([fib/1, howMany/1, howMany3d/1]).

fib(1) -> 0;
fib(2) -> 1;
fib(N) when N>0 ->
    fib(N-1) + fib(N-2).

%calling fib(4), calls fib(3) and fib(2)
%The call to fib(3) calls fib(2) and fib(1),
%which returns 1 and 0 respectively.
%The call to fib(2) on behalf of fib(4) returns
%1. So we have 1 + 1, which is the fourth number in
%the fibonnaci sequence.



howMany(0) -> 1;

howMany(N) when N > 0 ->
    N + (howMany(N-1)).

% how many 3d.
howMany3d(0) -> 1;
howMany3d3d(N) when N>0 ->
    howMany3d(N-1) + howMany(N-1).