InfiniteRunner Managers Can't be extended

GameManager.cs

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

namespace YakDogGames
{
    public class GameManager : MoreMountains.InfiniteRunnerEngine.GameManager
    {
        public override void LoseLives(int lives)
        {
            base.LoseLives(lives);
            YakDogGames.GUIManager.Instance.RefreshPoints();
        }
    }
}