DECLARE @FNumber INT,@fstartnumber INT ;

SET @fstartnumber=900

SELECT ROW_NUMBER() OVER (ORDER BY FNumber)+@fstartnumber-1 AS FIndex, a.FNumber

INTO #tmp

FROM(SELECT FNumber

     FROM t_Voucher

     WHERE FYear=2020 AND FPeriod=7 AND FGroupID=1

     UNION

     SELECT FNumber

     FROM t_VoucherBlankOut

     WHERE FYear=2020 AND FPeriod=7 AND FGroupID=1) a WHERE a.FNumber>=@fstartnumber;

IF NOT EXISTS (SELECT * FROM #tmp AS t WHERE t.FIndex<t.FNumber)

SELECT @FNumber=ISNULL(MAX(FIndex), 0)+1 FROM #tmp;

ELSE

    SELECT @FNumber=ISNULL(MIN(t.FIndex), 0)

    FROM #tmp AS t

    WHERE t.FIndex<t.FNumber;

DROP TABLE #tmp;

SELECT @FNumber;