四则运算

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 四则运算
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        public static  int right = 0;//正确的题目个数
        public static int mistake =0;//错误的题目个数
        //随机产生的随机数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;
        //调换四则运算的方法
        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;
            } 
           
        }
        
        //题目的正确个数的题目的错误个数
        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：助教你帮我看看我的代码哪里出错了，我感觉我的思路是对的啊，但是最后正确的题数和错误的题数结果不正确。
Psp分析:

psp	Personal	Software	Process	Stages	Time
planning	计划				15（m）
Estimate	估计这个任务需要多长时间				1(h)
Development	开发				2(h)
Analysis	需求分析				30(m)
Design Review	设计复审				40(m)
Design	具体设计				20(m)
Code review	代码复审				10(m)
Coding	具体编码				30(m)
Test	测试				1(m)