PL/SQL Functions Syntax in oracle

CREATE [OR REPLACE] FUNCTION [SCHEMA..] function_name
        [ (parameter [,parameter]) ]
        RETURN return_datatype
    IS | AS
        [declaration_section
		    variable declarations;
		    constant declarations;
        ]
	BEGIN
	    [executable_section
		    PL/SQL execute/subprogram body
        ]
	[EXCEPTION]
            [exception_section
		    PL/SQL Exception block
            ]
	END [function_name];