[Module] 关于module中public变量的疑问

Module var
    Implicit None
    integer,public:: n
    integer,private:: i,j
    contains
    subroutine cal(n)

    integer n              !  n变量如果定义能顺利运行，若不定义，会出现如下错误
                                !  This name does not have a type, and must have an explicit type.   [N]
                                !  但是，n已经在module中定义为public变量了，为啥不能直接用
    j=0
    do i=1,n
        j=j+i
        write(*,*)i,n,j
    end do
    end subroutine cal
end Module var

program main
    use var
    Implicit None
    n=10
    call cal(n)

    pause
end