how to Decrase Value Minus(-15,-30) count in C#

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C#

Grid view Minits minus permissionhrs=180 mints below Minis data

Late Mints Details
ProcessDate	LateIn	Mints
04 Mar 2014	20	15
05 Mar 2014	22	30
11 Mar 2014	18	15
20 Mar 2014	14	15
21 Mar 2014	30	30
22 Mar 2014	25	30
27 Mar 2014	26	30
28 Mar 2014	28	30

Permissionhrs minus minits Example
180-15 result is 165
165-30 result is 135
135-15 result is 120
120-15 result is 105
105-30 result is 75
75-30  result is 45
45-30  result is 15
15-30  result is -15

how to count howmany time -15 or anyone minusvalue came.

My Source Is code using but not came pls how to count minus(-)values came this grid
permissionhrs value is 180

 int halfday = 0;
int permissionResult = permissionhrs;
foreach (GridViewRow gvRows in gvLatemints.Rows)
{

    total2 = permissionResult - Convert.ToInt32(gvRows.Cells[2].Text.Trim());
    lblError.Text = "Total Leave :" + total2;
    permissionResult = permissionResult - Convert.ToInt32(gvRows.Cells[2].Text.Trim());
    if (permissionResult.ToString() == "-15")or anyone minus(-)value
    {
        halfday++;
        lblError.Text = "Total Leave :" + halfday;

    }

Thanks