you can accomplished that using vba.

Go to TOOLSMACROVisual Basic Editor

then click on the sheet and paste this code in the code area.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Cells(1, 3).Value = Cells(1, 3).Value + Cells(1, 1).Value
Cells(1, 3).Value = Cells(1, 3).Value - Cells(1, 2).Value
Cells(1, 1).Value = 0
Cells(1, 2).Value = 0
End Sub