Month number from Month Name in FCC

I am trying to derive month number from Month name (example, 1 for Jan, 2 for Feb etc.) using below member formula but not able to get the expected results when retried.

Is there any rule which I am breaking?

IF(@ISMBR("FCCS_Periodic"))
IF(@CURRMBR(Period)=="Jan")
1;
ELSEIF(@CURRMBR(Period)=="Feb")
2;
ELSEIF(@CURRMBR(Period)=="Mar")
3;
ELSEIF(@CURRMBR(Period)=="Apr")
4;
ELSEIF(@CURRMBR(Period)=="May")
5;
ELSEIF(@CURRMBR(Period)=="Jun")
6;
ELSEIF(@CURRMBR(Period)=="Jul")
7;
ELSEIF(@CURRMBR(Period)=="Aug")
8;
ELSEIF(@CURRMBR(Period)=="Sep")
9;
ELSEIF(@CURRMBR(Period)=="Oct")
10;
ELSEIF(@CURRMBR(Period)=="Nov")
11;
ELSEIF(@CURRMBR(Period)=="Dec")
12;
ENDIF;
ENDIF;