distance traveled unity script

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class DistanceTracker : MonoBehaviour {

    public float speed;

    public Text kmDisplay;

    Rigidbody rb;
    Vector3 input;

    Vector3 a;
    Vector3 b;

    bool takingA = true;

    float distance;

    void Start () {
        rb = GetComponent<Rigidbody>();
        distance = 0f;
    }

    void Update () {
        input = new Vector3(Input.GetAxis("Horizontal"), 0f, Input.GetAxis("Vertical"));
        rb.AddForce(input * speed * Time.deltaTime);

        if (takingA)
        {
            a = transform.position;
            takingA = false;
        }
        else
        {
            b = transform.position;
            takingA = true;
            distance += (a - b).magnitude;
        }
        kmDisplay.text = "Meters traveled: " + distance.ToString();
    }
}