noo I can declare the global variable

You can declare it perfectly.

What you can not do is initialize it with the value of another variable.

Global variables can only be initialized with constant values. D1 = 2 is correct. D2 = D1 - 2 is incorrect , since you reference another variable.

For these cases, macros of the preprocessor are usually used:

#define VALOR_CONSTANTE 2

int D1 = VALOR_CONSTANTE;
int D2 = VALOR_CONSTANTE - 2;

The macros are not code C; your processed will perform before the compiler starts its work. Therefore, when the compiler sees the code, what it really sees is:

int D1 = 2;
int D2 = 2 - 2;

There, there is no reference to other variables, and it is compiled without problems.

EDITO

The modifier const , applicator to the declaration of variables , is used to prevent that we change the value of a variable, after initializing it:

const int a = 10;

int main( void ) {
  a = 20; // <- ERROR !!
  ...

It's a help for the developer, but a follows as a variable, not a constant value >.

const int a = 10;
int b = a - 5; // <- ERROR. a es una variable, no una expresión constante.

To declare global in C and use it in the way you are showing (using constant values in your declaration) you will need the clause define , here I leave an example:

#include <stdio.h>

#define D1 5 // No requiere definir el tipo
int D2 = D1 - 2; // Requiere especificar el tipo

int main(int argc,char* argv[])
{
    printf("Valor de D2: %d\n", D2);
    return 0;
}

For this case you must use define which is a preprocessor statement, that is, its value is assigned to variable D1 as the first step when compiling the complete program, in the way you were doing before , when assigning the value of D2 the compiler would throw an error because it could not obtain the value of D1 since this was not defined at that time.

I hope I have helped you, regards!