Use Fortran dll in VC++

Hi! I created a Fortran code with 3 function.  This is my Fortran code:

real function circle_area(radius)
!DEC$ ATTRIBUTES DLLEXPORT :: CIRCLE_AREA
!DEC$ ATTRIBUTES ALIAS : "Circle_Area" :: CIRCLE_AREA
    
        implicit none
       real radius
       real, parameter :: PI = 3.14159
        circle_area = radius*radius*PI
       return

end function

integer function sum(a)
!DEC$ ATTRIBUTES DLLEXPORT :: SUM

     implicit none
     integer :: a(10)
      integer i
      sum=0
     do i=1,10
      sum=sum+a(i)
     end do
      return

end function

subroutine MakeLower(string)
!DEC$ ATTRIBUTES DLLEXPORT :: MAKELOWER

      implicit none
      character(len=*) :: string
      integer :: len, i, code
      len = len_trim(string)
      do i=1,len
        code = ichar(string(i:i))
        if ( code >= ichar('a') .and. code <= ichar('z') ) then
          string(i:i) = char(code-32)
        end if
      end do
      return

end subroutine

 I use this Fortran code to create dll (Dll1). Then I wrote a c program to call function  circle_area() in Fortran code.

#include "stdafx.h"
#include "stdlib.h"
extern "C" {float circle_area(float *a); }

int _tmain(int argc, _TCHAR* argv[])
{
    float b = 3.;
    circle_area(&b);
    system("pause");
    return 0;
}

Before I ran my c code, I included my dll in Visual Studio 2013. First I added my Dll1.lib directory in VC++ directories-> Library Directories,  in Linker -> General -> Additional Library Directories  added Dll1.lib directory again,  in Linker -> Input -> Additional Dependencies added Dll1.lib. Last, I put Dll1.dll in c project folder that it will create .exe together. 

After running my c code, I got these messages :

error LNK2019: unresolved external symbol '_circle_area ' referenced in function '_wmain'

error LNK1120: 1 unresolved external symbol

It seems that I need  something setup. Can someone help me?