Check order of occurence, ignoring repeatation and assign a value in new coloumn

I have a processed dataset that looks like this. The S.No column follows a certain order, with some values repeating as in the sample below.

I want checkand obtain a result, where if the the rows follow the sequence as in increasing order, 1-2-3-4-5- etc (repeatation ignored), then a new coloumn needs to be assigned with a value as “X” and if it follows otherwise, that is 5-4-3-2-1, then assign “Y”.

S. No Name

1 A

1 A

1 A

1 A

2 B

2 B

3 C

3 C

3 C

4 D

4 D

5 E

5 E