Function compile error

Hello,
I am trying to create the function as below but it is throwing error
'ERROR: syntax error at or near "DECLARE"', Could some one help me please

CREATE FUNCTION check_password(databasename text, tablename text,
indexname text)RETURNS VOID AS
DECLARE v_count INTEGER;
BEGIN
select count(1) into v_count from pg_index inx where
inx.indexrelid in
(select oid from pg_class where relname=$3 and relowner in
(select oid from pg_authid where rolname=$1))
and inx.indrelid in
(select oid from pg_class where relname=$2 and relowner in
(select oid from pg_authid where rolname=$1));
if v_count = 0 then
execute immediate 'create unique index $3 on $2 (acn_id)';
end if;
END;

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