How to read return value from .EXE

Question

Can I get some help with getting return value from .EXE in another assembly. I am getting the following error:

Error 1:Program 'Sample.exe' does not contain a static 'Main' method suitable for an entry point sample.exe.

Customer.cs

public int InsertCustomer() {
    return 100;
}

Program.cs

public static int Main() {
    customer x = new customer ();
    int ID_c = InsertCustomer();
    return ID_c;
}