input was not in correct format

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MS-Access

int sum = 0;

for (int i = 0; i < dataGridView1.Rows.Count; ++i)

{
               
 sum += Convert.ToInt32(dataGridView1.Rows[i].Cells[38].Value);
            
}
            
txt_balance.Text = sum.ToString();
            
txt_total.Text = sum.ToString();
            
txt_Exempt.Text = sum.ToString();

am having a error like Input string is not in correct format. i want calcuate the column values if datagridview row is incremented. i tried when i execute this am getting error

pls guide...