Doubt about formula transcription

hey guys,

im with a little problem when im transcripting a math formula from
maple to fortran, because when i put a solution on the equation, i got
a result diferent from zero.

The formula is: 2*arctan(sin(a)*cos(a)/(.8-cos(a)^2))-4/9*Pi ;
I've transcripted to fortran as: 2*atan((sin(a)*cos(a))/(0.8-
cos(a)**2))-((pi)*4/9)

There's correct?!
a piece of my code:

! usando o metodo da zoada kkkkk
program missel
!constantes e variaveis
real FUNCAO
! programa
write(*,*)'Valor de f:',FUNCAO(1.023762140);
end
! funcoes, para facilitar, tava foda ficar repetindo...
real function FUNCAO(a);
real pi,a; parameter(k=0.8,pi=3.141592653);
FUNCAO=dble(2*atan((sin(a)*cos(a))/(k-cos(a)**2))-((pi)*4/9))
return
end
.