I've made a nice code that works but why does my character keep floating away instead of moving and staying in one place this is my code.

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

public class NewBehaviourScript : MonoBehaviour

$\{$

const float speed $=1\mathrm{.}0$f;

Vector$3$ velocity;

$//$ Start is called before the first frame update

void Start$()$

$\{$

velocity $=$ speed $*$ new Vector$3(1\mathrm{.}0$f$,1\mathrm{.}0$f$,0\mathrm{.}0$f$)$;

$\}$

$//$ Update is called once per frame

void Update$()$

$\{$

if $($Input$.$GetKey$("$w$"))$

$\{$

velocity $=$ new Vector$3($velocity$.$x$,1*$ speed, $0\mathrm{.}0$f$)$;

$\}$

else if $($Input$.$GetKey$("$a$"))$

$\{$

velocity $=$ new Vector$3(-1*$ speed, velocity.y$,0\mathrm{.}0$f$)$;

$\}$

else if $($Input$.$GetKey$("$s$"))$

$\{$

velocity $=$ new Vector$3($velocity$.$x$,-1*$ speed, $0\mathrm{.}0$f$)$;

$\}$

else if $($Input$.$GetKey$("$d$"))$

$\{$

velocity $=$ new Vector$3(1*$ speed, velocity.y$,0\mathrm{.}0$f$)$;

$\}$

transform.position $=$ transform.position $+($velocity $*$ Time.deltaTime$)$;

$\}$

$\}$