try this in OK button click
Code:
Dim I As Integer, War As String, Sum As Double
intNext = Text2.Text
    
    For I = 1 To intNext
Again:
        War = InputBox("Added   " & I & "   value", "Adding sections")
        If War = "" And Sum <= 0 Then Exit Sub
        If War = Cancel Then
            If MsgBox("You have incomplete value or you want to break?", _
             vbYesNo + vbQuestion, "Question") = vbYes Then
                          Exit Sub
           Else
             GoTo Again
           End If
        
        End If
        
        If IsNumeric(War) = False Then
            MsgBox "Enter with correct an value", vbCritical
            GoTo Again
        End If
       Sum = Sum + CDbl(War)
    Next
    Text1.Text = CStr(Sum)