How to call c++ dll in wcf Service?

I have written simple method for math function in c and create dll as mathfunction.dll

bellow cose is mathfunction.dll.

#include

extern "C"

{

__declspec(dllexport) int add(int a,int b)

{

return a+b;

}

__declspec(dllexport) int subtract(int a,int b)

{

return a-b;

}

I try to consume mathfunction.dll in wcf service then throw error message.

'An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)'

Bellow code used in wcf service.

public int Sum(int value, int value1)

{

return add(value, value1);

}

[DllImport("MathFunction.dll", CallingConvention = CallingConvention.Cdecl)]

private static extern int add(int num1, int num2);