SQL expression cannot be converted to CS Expression

I have a column "YEAR_MONTH" has the format 'YYYY.MM'

I tried to use a calculated column to extract the month

In SQL Language

     to_int(left("YEAR_MONTH",4))

     error "SQL Expression cannot be converted to CS Expression. Reason: Unregistered function Name: "left"

In CE langauge

    int(leftstr("YEAR_MONTH",4))

    error "Attribute value is not a number"

HANA version 1.00.102. Execute in SQL engine is selected. Any thought on why? Many thanks