四則運算

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 四則運算
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        public static  int right = 0;//正确的題目個數
        public static int mistake =0;//錯誤的題目個數
        //随機産生的随機數0-10
        private void RandomNum()
        {
            Random ran = new Random();
            int n1, n2;
            n1 = ran.Next(0, 11);
            n2 = ran.Next(0, 11);
            textBox1.Text = n1.ToString();
            textBox2.Text = n2.ToString();
            textBox3.Text = "";
            
        }       
        public static int sum;
        //調換四則運算的方法
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            
            textBox3.Focus();
            string str = comboBox1.SelectedItem.ToString(); 
            RandomNum();
            switch (str)
            {
                case "+":
                    sum = int.Parse(textBox1.Text) + int.Parse(textBox2.Text);
                    break;
                case "-":
                    sum = int.Parse(textBox1.Text) - int.Parse(textBox2.Text);
                    break;
                case "*":
                    sum = int.Parse(textBox1.Text) * int.Parse(textBox2.Text);
                    break;
                case "/":
                    sum = int.Parse(textBox1.Text) / int.Parse(textBox2.Text);
                    break;
            } 
           
        }
        
        //題目的正确個數的題目的錯誤個數
        private void button3_Click(object sender, EventArgs e)
        {
            textBox3.Focus();
            RandomNum();
            if(textBox3.Text == sum.ToString())
            {
                right++;
                RandomNum();
            }
            else
            {
                mistake++;
                RandomNum();
            }
            
        }
        private void button1_Click(object sender, EventArgs e)
        {
            textBox4.Text = right.ToString();
            textBox5.Text = mistake.ToString() ;
        }
    }
}
求助：助教你幫我看看我的代碼哪裡出錯了，我感覺我的思路是對的啊，但是最後正确的題數和錯誤的題數結果不正确。
Psp分析:      

psp	Personal	Software	Process	Stages	Time
planning	計劃	15（m）
Estimate	估計這個任務需要多長時間	1(h)
Development	開發	2(h)
Analysis	需求分析	30(m)
Design Review	設計複審	40(m)
Design	具體設計	20(m)
Code review	代碼複審	10(m)
Coding	具體編碼
Test	測試	1(m)