Sub silnia()

Dim n, silnia As Integer
Dim r As Range

Set r = Range(2, 1)

n = Cells(2, 1).Value

If n > 0 Then
silnia = n * (n - 1)

ElseIf n = 0 Then
silnia = 1

ElseIf n < 0 Then
MsgBox ("Podales ujemna liczbe")

ElseIf IsEmpty(r) Then
MsgBox ("Nie wprowadziles liczby")

ElseIf IsNumeric(r) = False Then
MsgBox ("Wprowadziles litere")

End If

MsgBox silnia

End Sub