Public Class Form1

    Private Sub btnSum_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSum.Click
        Dim num As Double = 0
        Dim sum As Double

        lstpos.Items.Clear()
        Do While num <> -1

            num = CDbl(InputBox("Input a positive number(enter -1 when finished)"))
            If num >= 0 Then

                num = num + sum
                sum = num
            ElseIf num = -1 Then

                lstpos.Items.Add(sum)

            Else
                MsgBox("Not a positive number")
            End If


        Loop


    End Sub
End Class