Fortran Grade Calculator

s!Grade calculator
program GradeCalc
    implicit none
    integer :: score

    !prototype function return length
    character*2 calc

    !user inputs score
    write (*,*) '  Enter score: '
    read (*,*) score

    !show grade
    write (*,*) '    Your grade: ', calc(score)

end program GradeCalc

!converts from numerical to letter grade
function calc(sco) result(gr)
    implicit none

    character(*) :: gr
    integer :: sco

    SELECT CASE (sco)
        CASE (90:100)
            gr= 'A'
        CASE (80:89)
            gr= 'B'
        CASE (70:79)
            gr= 'C'
        CASE (60:69)
            gr= 'D'
        CASE (0:59)
            gr= 'F'
        CASE DEFAULT
            write (*,*) '    ERROR: Invalid score '
            gr= 'x'
    END SELECT
end function calc