ADO.NET中总是提示输入字符串的格式不正确（求大神帮忙改正错误）

private void button3_Click(object sender, EventArgs e)
        {           
            student.glpwd  =  watermarkTextBoxGLPWD.Text;//管理员密码
            SqlCommand cmd = new SqlCommand();
            try
            {
                student.con.Open();
                cmd.Connection = student.con;
                = "select * from adminpwd where 密码='" + student.glpwd  + "'";
                int n = Convert.ToInt32(cmd.ExecuteScalar());
                student.con.Close();
               if (n <= 0)
                {
                    MessageBox.Show("密码输入错误！");
                }
                else
                {
                    admin1 f4 = new admin1();//第四个窗体（管理员）
                    this.Hide();
                    f4.ShowDialog();
                }
               

[此贴子已经被作者于2018-6-7 10:58编辑过]