factorial of a number using erlang

Factorial of a non negative Number in math represented as n!.
for example:-
5!= 5x4x3x2x1=120

-module(my_math).
-export([factorial/1]).

factorial(Num) when is_integer(Num) and (Num >= 0) ->
        factorial(Num, 1).
factorial(0, Res)->
        Res;
factorial(Num, Res)->
        factorial(Num -1, Num*Res).

Compile and Run

rajesh@ideapad:~/Rajesh/Blog/factorial$ erl
Erlang/OTP 18 [erts-7.3] [source] [64-bit] [smp:4:4] [async-threads:10] [kernel-poll:false]

Eshell V7.3  (abort with ^G)
1> c(my_math).
{ok,my_math}
2> my_math:factorial(5).
120
3> my_math:factorial(0).
1
4> my_math:factorial(1).
1
5> my_math:factorial(10).
3628800
6>