How to do numeric sorting in pivot grid

C# 

this.pivotGrid1.PivotRows.Add(new PivotItem { FieldHeader = "Quantity", FieldMappingName = "Quantity", TotalHeader = "Total", Comparer = new QuantityComparer() });

public class QuantityComparer : IComparer
{
     public int Compare(object x, object y)
     {
          if (x == null && y == null)
              return 0;

          if (x == null)
              return -1;

          if (y == null)
              return 1;

          if (Int32.Parse(y.ToString()) == Int32.Parse(x.ToString()))
              return 0;

          return Int32.Parse(y.ToString()) < Int32.Parse(x.ToString()) ? 1 : -1;
     }
}

Figure: pivot grid with numeric sorting