ADD_Script.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System.Runtime.InteropServices;

public class ADD_Script : MonoBehaviour {
    [DllImport("ADD")]
    private static extern int ADD_2(int A, int B);

    private readonly int _A = 1;
    private readonly int _B = 2;

	// Use this for initialization
	void Start () {
        GetComponent<TextMesh>().text = ADD_2(_A, _B).ToString();
	}
	
	// Update is called once per frame
	void Update () {
		
	}
}