Sub test()
Dim fin&, i&, temp
fin = Cells(Rows.Count, 1).End(xlUp).Row
Range("c2").ClearContents
On Error Resume Next
x = 2
For i = 2 To fin
x = x + 1
If Cells(i, 1) = Cells(x, 1) - 1 Then
temp = temp & Cells(x - 1, 1) & " à "
Else
temp = temp & Cells(i, 1) & ","
End If
Next i
Range("c2") = temp
End Sub