Problem With Undo & Redo Option

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C#

Hey
i have created a project that draw an Ellipse via movement of the mouse by the user
and 2 buttons Undo & Redo

The undo works o.k.
but i have a problem with the Redo option

if i draw 3 circles
and then click twice on the undo button i delete 2 circles and left with the first circle
which is great
but when i click the redo button once i want to return the second circle that i draw
but i get the insted the 2 circle (2th and 3th and final)

here is my code

namespace Final_Project___Undo_and_Redo
{
    public partial class Form1 : Form
    {
        private Bitmap PicInMemory; 
        private Graphics PaintOnMyPic;
        private int FirstX, FirstY;
        private Graphics PaintOnPanel;
        List<bitmap> UndoList;
        List<bitmap> RedoList;
        private bool UndoMade = false;
        private bool RedoMade = false;

        public Form1()
        {
            InitializeComponent();
            PicInMemory = new Bitmap(panel1.Width, panel1.Height);
            PaintOnPanel = panel1.CreateGraphics();
            PaintOnMyPic = Graphics.FromImage(PicInMemory);
            PaintOnMyPic.Clear(Color.White);
            UndoList = new List<bitmap>();
            RedoList = new List<bitmap>();
        }

        private void panel1_MouseDown(object sender, MouseEventArgs e)
        {
            FirstX = e.X;
            FirstY = e.Y;
        }

        private void panel1_MouseMove(object sender, MouseEventArgs e)
        {
            Pen MyPen = new Pen(Color.Aqua);
            if (e.Button == MouseButtons.Left)
            {
                    PaintOnPanel = panel1.CreateGraphics();
                    PaintOnPanel.DrawImage(PicInMemory, 0, 0);
                    PaintOnPanel.DrawEllipse(MyPen, FirstX, FirstY, e.X - FirstX, e.Y - FirstY);
            }
        }

        private void panel1_MouseUp(object sender, MouseEventArgs e)
        {
            UndoList.Add(new Bitmap(PicInMemory));
            Pen MyPen = new Pen(Color.Aqua);
            PaintOnMyPic = Graphics.FromImage(PicInMemory);
            PaintOnMyPic.DrawEllipse(MyPen, FirstX, FirstY, e.X - FirstX, e.Y - FirstY);
            PaintOnPanel = panel1.CreateGraphics();
            PaintOnPanel.DrawImage(PicInMemory, 0, 0);
            RedoList.Add(new Bitmap(PicInMemory));
        }

        private void BtnUndo_Click(object sender, EventArgs e)
        {
            if (UndoList.Count == 0)
            {
                BtnUndo.Enabled = false;
                return;
            }
            PicInMemory = UndoList[UndoList.Count - 1];
            PaintOnPanel = panel1.CreateGraphics();
            PaintOnPanel.DrawImage(PicInMemory, 0, 0);
            UndoList.RemoveAt(UndoList.Count - 1);
            UndoMade = true;
        }

        private void BtnRedo_Click(object sender, EventArgs e)
        {
            if (RedoList.Count == 0 || UndoMade == false)
            {
                BtnRedo.Enabled = false;
                return;
            }
            PicInMemory = RedoList[RedoList.Count - 1];
            PaintOnPanel = panel1.CreateGraphics();
            PaintOnPanel.DrawImage(PicInMemory, 0, 0);
            UndoMade = false;
        }
    }
}

Thanks for the help