为什么第一次点减运算能运算第二次之后点其他运算符后一直加怎么修改，新手帮下哈

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 SimpleCalculator
{
public partial class frmCalculator : Form
{
private int left = 0;
private int right =0;
private string oper = "+";
private bool isOperator = false;

public frmCalculator()
{
InitializeComponent();
}

private void btnEqual_Click(object sender, EventArgs e)
{
try
{
switch (oper)
{
case "+":
left = left + right;
break;
case "-":
left = left - right;
break;
case "*":
left = left * right;
break;
case "/":
left = left / right;
break;
default:
break;
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
tbShow.Text = Convert.ToString(left);
right = left;
isOperator = false;
oper = "";
}

private void btnNum_Click(object sender, EventArgs e)
{
Button btn = (Button)sender;
if (isOperator)
{
right +=Convert.ToInt32( btn.Text);
tbShow.Text = right.ToString();
}
else
{
left +=Convert.ToInt32( btn.Text);
tbShow.Text = left.ToString();
}

}

private void btnOperator_Click(object sender, EventArgs e)
{
Button btn = (Button)sender;
oper = btn.Text;
isOperator = true;
}
}