ZwTerminateProcess failed with [DllImport]

[escap3]

I just want to use ZwTerminateProcess() native function through my C# Application.
I've been written this part of code :

[DllImport("ntdll.dll")]
        public static extern NtStatus ZwTerminateProcess(
            [In]    [Optional] IntPtr ProcessHandle,
            [In]    NtStatus ExitStatus
            );

but the compiler issues some error code :

Error	2	The type or namespace name 'NtStatus' could not be found (are you missing a using directive or an assembly reference?)

how Can I import this function ?