Add acceleration to movement

Hey guys, I want to have a customizable acceleration, but i really don't know how i can do that.

Hope someone knows how I can do that

Here's the movement script:

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

public class MenschMovement : MonoBehaviour { //Variables public CharacterController controller; public float gravity = -9.81f; public Transform groundCheck; public float groundDistance = 0.4f; public LayerMask groundMask; public float jumpHeight = 3f; public float speed;

Vector3 velocity; bool isGrounded; private bool canDoubleJump;

void Update() {

   //Gravity & GroundCheck
   isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

   if(isGrounded && velocity.y < 0)
   {
     velocity.y = -4f;
   }


   //Movement
   float x = Input.GetAxisRaw("Horizontal");
   float z = Input.GetAxisRaw("Vertical");

    Vector3 move = transform.right * x + transform.forward * z;


   velocity.y += gravity * Time.deltaTime;
   controller.Move(move * speed * Time.deltaTime);
   controller.Move(velocity * Time.deltaTime);


   //Jump / DoubleJump
   if(isGrounded)
   {
     canDoubleJump = true;
   }

   if(Input.GetButtonDown("Jump"))
   {
     if (isGrounded) {
     velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
   } else {
     if (canDoubleJump == true) {
     velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
     canDoubleJump = false;
     }
   }


 }

} }