Timer that increases every time I kill an enemy

Well, I have a countdown script so that when it ends the scene changes:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using UnityEngine.SceneManagement;
 
 public class GameOver : MonoBehaviour
 {
     public string levelToLoad;
     public float timer = 10f;
     private Text timerSeconds;
 
     void Start ()
     {
         timerSeconds = GetComponent<Text>();
     }
 
     void Update()
     {
         timer -= Time.deltaTime;
         timerSeconds.text = timer.ToString("f2");
         if (timer <= 0)
         {
             SceneManager.LoadScene(0);
         }
     }
 }

I have another script for enemies dead:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class DamageManager : MonoBehaviour
 {
 
     public GameObject[] deadbody;
     public AudioClip[] hitsound;
     public int hp = 100;
     public int Score = 10;
     
     private Vector3 velositydamage;
     private float distancedamage;
     
     void Start(){
         
     }
     
     void Update(){
         if (hp <= 0) {
             Dead (Random.Range (0, deadbody.Length));
         }
     }
     
     public void ApplyDamage (int damage, Vector3 velosity, float distance)
     {
         if (hp <= 0) {
             return;
         }
         distancedamage = distance;
         hp -= damage;
         velositydamage = velosity;
     }
     
     public void ApplyDamage (int damage, Vector3 velosity, float distance, int suffix)
     {
         if (hp <= 0) {
             return;
         }
         distancedamage = distance;
         hp -= damage;
         velositydamage = velosity;
         if (hp <= 0) {
             Dead (suffix);
         }
         
     }
     
     public void AfterDead (int suffix)
     {
         int scoreplus = Score;
         
         if(suffix == 2){
             scoreplus = Score * 5;    
         }
             
         ScoreManager score = (ScoreManager)GameObject.FindObjectOfType (typeof(ScoreManager));    
         if(score){
             score.AddScore (scoreplus, distancedamage);
         }
     }
     
     
     // ** Important! for Ragdoll replacement
     private AS_RagdollReplace ragdollReplace;
     
     public void Dead (int suffix)
     {
 
         if (deadbody.Length > 0 && suffix >= 0 && suffix < deadbody.Length) {
             // this Object has removed by Dead and replaced with Ragdoll. the ObjectLookAt will null and ActionCamera will stop following and looking.
             // so we have to update ObjectLookAt to this Ragdoll replacement. then ActionCamera to continue fucusing on it.
             GameObject deadReplace = (GameObject)Instantiate (deadbody [suffix], this.transform.position, this.transform.rotation);
 
             ragdollReplace = deadReplace.GetComponent<AS_RagdollReplace> ();
             // copy all of transforms to dead object replaced
             CopyTransformsRecurse (this.transform, deadReplace);
             // destroy dead object replaced after 5 sec
             Destroy (deadReplace, 5);
             // destry this game object.
             Destroy (this.gameObject,1);
             this.gameObject.SetActive(false);
         
         }
         AfterDead (suffix);
     }
     
     // Copy all transforms to Ragdoll object
     public void CopyTransformsRecurse (Transform src, GameObject dst)
     {
         
     
         dst.transform.position = src.position;
         dst.transform.rotation = src.rotation;
 
         
         foreach (Transform child in dst.transform) {
             var curSrc = src.Find (child.name);
             if (curSrc) {
                 CopyTransformsRecurse (curSrc, child.gameObject);
             }
         }
     }
 
 }
 

How can I make that every time I kill an enemy, 10 seconds are added to the timer?