error in if mupad file

hello all,my code is

if true
c:=input(c)
d:=input(d)
  if d:=c
then
Ml:=ro*PI*(c^2)*l/2
elif  d>c then teta0:=arccos(d-c) 
Ml:=ro*(c^2)*l*(2*PI-2*teta0+sin(2*teta0))/2
elif d<c then  teta0:=acos(1-h) 
Ml:=ro*(c^2)*l*(2*teta0-sin(2*teta0))/2
end_if:
end
Error: Unexpected 'identifier'. [line 5, col 1]
could any body help me.
with regards