Mouse move to control the animation

Hello! I do not know much English, so I'm sorry for the mistakes: D I'm doing a 2D platformer in which you control your character with the mouse. I need to move in the Y-axis to less than "a" up to play its one animation down - another, when you stop to play its third, and moving on more than "a" to play its fourth. Please help me write a script! I would be very grateful :) Here's the script:

using UnityEngine; using System.Collections;

public class MouseControl : MonoBehaviour {

 void Start () {}
 public float speed=0.1f;
 Vector3 MousePos=Vector3.zero;
 void Update () {
 MousePos = new Vector3(speed, Input.GetAxis("Mouse Y"), 0);
 transform.position+=MousePos;
 }

}