cant convert to int

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

namespace studentCalc_10v_17
{
    public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();
            DataGridView dgv1 = dataGridView1;
            DataGridView dgv2 = dataGridView2;

            dgv1.Rows.Add(student);
            dgv1[0, 0].Value = "Иван Петров";
            dgv1[1, 0].Value = 6;
            dgv1[2, 0].Value = 6;
            dgv1[3, 0].Value = 6;



            dgv2.Rows.Add(id);
            dgv2.Rows.Add();
            dgv2[0,0].Value = "БЕЛ";
            dgv2[0,1].Value = "Математика";
            dgv2[0,2].Value = "Информатика";

        }

        private void dataGridView2_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {

        }

        private void addData_Click(object sender, EventArgs e)
        {
            DataGridView dgv1 = dataGridView1;
            debug.Text = dgv1.Rows.Count.ToString();
            for (int i = 0; i < dgv1.Rows.Count; i++)
            {
                int sum = 0;

                for (int j = 0; j < dgv1.Columns.Count; j++)
                {
                    sum += int.Parse(dgv1[j, i].Value.ToString()); // the line that makes the problem
                    double averageStud = (double)sum / dgv1.Rows.Count;
                    dgv1[dgv1.Columns.Count + 1, i].Value = Math.Round(averageStud, 2);
                }
            }
        }

        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            int numRow = e.RowIndex;
            int numCol = e.ColumnIndex;
            row.Text = " Ред: " + numRow.ToString();
            col.Text = " Колона: " + numCol.ToString();
        }
    }
}