Run If statement based on SheetName

I'm trying to run an If statement that can only run if the worksheet exists.
Any help would be much appreciated.

Below is my code:

If Worksheets(SheetName).Name = "October" Then
 
    Sheets("Gene").Select
    Range("C4").Select
    ActiveCell.FormulaR1C1 = "=SUMPRODUCT(--(July!R2C4:R20000C4=""IFC-00546492-TF055451""),--(July!R2C2:R20000C2=""Staff Cost""),July!R2C5:R20000C5)"
    Else If...
.
.
.
End If