private void button2_Click(object sender, EventArgs e)
    {
        int grandTotal = CalculateNodes(this.treeView1.Nodes);
    }
    private int CalculateNodes(TreeNodeCollection nodes)
    {
        int grandTotal = 0;
        foreach (TreeNode node in nodes)
        {
            if (node.Nodes.Count > 0)
            {
                int childTotal = CalculateNodes(node.Nodes);
                node.Text = childTotal.ToString();
                grandTotal += childTotal;
            }
            else
            {
                grandTotal += Convert.ToInt32(node.Text);
            }
        }
        return grandTotal;
    }

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Hope It Will Help You