how to implement adding of numbers in PROLOG

write a knowledge base add.pl

with the following rules:

// for two variables 

sum(X,Y):-

    S is X+Y,

   write('sum of '),write(X),write('and '),write(Y),write('is ='),write(S).

add:-

    write_ln('Enter the first number:-'),

    read(First),

    write_ln('Enter second number:-'),

    read(Second),

    S is First+Second,

     write('sum of '),write(First),write('and '),write(Second),write('is ='),write(S).

now we can close this windows and in the main WINDOW

either type  ['add.pl'].

or consult the add.pl file.

now you can call this funcation via two ways :-

?- sum(5,44).

it will give output as 

sum of 5 and 44 is 49

or you can call via 

add.

Enter the first number .

:| 45.

Enter the second Number .

:| 45.

sum of 45 and 45 is equal to 90.

note i have used sum:- instead of add:- so i have to call it via sum.