数组a中有20个正整数，以降序排序。在文本框Text1中输入一个正整数data，寻找数组中是否存在两个元素之差等于给定的数据。若是则输出这两个数（有多对数满足条件，只输出最先找到的数对）。否则输出“not find”。实现此问题的算法程序如下，加框处代码有误，请改正。
①________________________
②________________________
③________________________
Private Sub Command1_Click()
Const n = 20
Dim a(20) As Integer
Dim data As Integer, left As Integer, right As Integer, m As Integer
Dim f As Boolean
f = True
data = Val(Text1.Text)
' 随机产生符合题意的数组a中各元素并按降序排序
For i = 1 To n - 1

①

right = n

Do While ②

m = (left + right) \ 2
If a(i) - a(m) > data Then
right = m - 1
ElseIf a(i) - a(m) < data Then
left = m + 1
Else
Text2.Text = a(i) & a(m)
③
End If
Loop
Next i
If f=True Then Text2.Text = "not find"
End Sub