If each slide increases (or decreases depending on what is needed) the global total variable as well as its own, then the global variable will be a sum of all slides.
An alternative would be to loop though all slides and read the label values.

You could insert a Module and create:
Sub OnSlideShowPageChange(ByVal SSW As SlideShowWindow)

    Dim totalBoysScore As Integer
    Dim totalGirlsScore As Integer


    Dim sld As Slide
    Dim shp As Shape
    
    For Each sld In ActivePresentation
        
        For Each shp In sld.Shapes
            If shp.Name = "Label11" Then
                
                If sld.Name = "Slide41" Then
                    ' this is the last slide, put the totals in the labels
                    shp.TextFrame.TextRange.Text = totalBoysScore
                Else
                    totalBoysScore = totalBoysScore + Val(shp.TextFrame.TextRange.Text)
                End If
                
            ElseIf shp.Name = "Label12" Then
            
                If sld.Name = "Slide41" Then
                    ' this is the last slide, put the totals in the labels
                    shp.TextFrame.TextRange.Text = totalGirlsScore
                Else
                    totalGirlsScore = totalGirlsScore + Val(shp.TextFrame.TextRange.Text)
                End If
                            
            End If
        Next shp
    Next sld



End Sub
This event runs every time a slide is, well, changed during the presentation. This isn't completely tested just trying to give an alternate idea