Compilation Fail in DecreaseHealth Method.

using System;

namespace TreehouseDefense
{
    class Game
    {
        //static because this can call without create object
        private static void Main()
        {
            //Declare variable map with type Map and assign with newMap object.
            //this new object need values for it to be created
            Map map = new Map(8, 5);

            try
            {
                //Pass MapLocation array into path constructor
                Path path = new Path(
                    new [] {
                        new MapLocation(0, 2, map),
                        new MapLocation(1, 2, map),
                        new MapLocation(2, 2, map),
                        new MapLocation(3, 2, map),
                        new MapLocation(4, 2, map),
                        new MapLocation(5, 2, map),
                        new MapLocation(6, 2, map),
                        new MapLocation(7, 2, map)
                    }
                );



            }
            //Start with most specific
            catch(OutOfBoundsException ex)//ex is an object of OutOfBoundsException, by declare it
            {
              //access message inside ex
                Console.WriteLine(ex.Message);
            }
            catch(TreehouseDefenseException)
            {
                Console.WriteLine("Unhandled TreehouseDefenseException");
            }
            //To the most general
            catch(Exception ex)//This exception can handle all the exception that occured, because it main
            {
                //There are infomation inside ex variable so we can use them
                Console.WriteLine("Unhandled Exception: " + ex);
            }
        }
    }
}

namespace TreehouseDefense
{
    class Invader
    {
        //keep track, where the invader is
        private int _pathStep = 0;
        //Use to store path instance
        private readonly Path _path;


        //Public property is better, bacause we shold make field private
        public int Health { get; private set; } = 2;

        //Auto-Property, no backup field but still can use get and set
        public MapLocation Location
        {
            get
            {
                return _path.GetLocationAt(_pathStep);
            }
        }

        //Tell whether the invader is reached the end of the path or not
        public bool HasScored
        {
            get
            {
                //Check with Path.Length Property
                return _pathStep >= _path.Length;
            }
        }

        //Use for Check if invader is death or not for other class to know
        public bool IsDeath
        {
            get
            {
                //Check with Path.Length Property
                return Health == 0;
            }
        }


        public bool IsActive
        {
            get
            {
                //If return false means Death, or Reached the end. True means isn't Death, or isn't reach the end. 
                return !(IsDeath || HasScored);
            }
        }

        //Constructor for initialize the LocationProperty
        public Invader(Path path)
        {
            _path = path;
        }

        //Use this method to move the invader
        public void Move()
        {
            _pathStep += 1;
        }

        public int DecreaseHealth(int factor)
        {
            return Health -= factor;
        }

    }
}