SSRS report sum on visible expression value on Table

Hi Experts,

I am trying to sum of visible expression Total days diff = 136+15+13=164

Total days diff expression is =DateDiff("d",Fields!PRWFDate.Value,Fields!POWFDate.Value)

Total days diff expression for last row is =SUM(IIf(DateDiff("d", Fields!PRWFDate.Value, Fields!POWFDate.Value) <= 0,
DateDiff("d", Fields!POWFDate.Value, Fields!PRWFDate.Value),
DateDiff("d", Fields!PRWFDate.Value, Fields!POWFDate.Value)))

giving value 43596446.

I am trying to sum Total days diff = 136+15+13=164 than divide number of PO Id which is 3 than Average = (164/3)=54.66

The value 17.67 = 53/3

Thanks,

Faiz