下列VB程序运行时，单击命令按钮Command1后，标签框Label2中显示的内容是________
Private Sub Command1_click()
Dim s as integer,I as integer
S=0
For i="1" to 20 step 3
s=s+(-1)^i*i
Next i
Label1.caption=str(i)
Label2.caption=str(s)
End Sub