Program Example

    PROGRAM Main
     IMPLICIT NONE
     REAL :: x
      READ*, x
      PRINT*, FLOOR(x) ! Intrinsic
      PRINT*, Negative(x)
    CONTAINS
     REAL FUNCTION Negative(a)
      REAL, INTENT(IN) :: a
       Negative = -a
     END FUNCTION Negative
    END PROGRAM Main

For more information, click here