Вычислить размер премии- QBasic(Бейсик)

INPUT "vvedite oklad ", ok
INPUT "vvedite plan ", plan
INPUT "skolko vypolneno? ", vyp
proc = (vyp - plan) / plan * 100
SELECT CASE proc
    CASE IS <= 0
        PRINT "plan ne vypolnen, premii ne budet"
    CASE IS <= 10
        pr = 0.01 * ok
    CASE IS <= 20
        pr = 0.015 * ok
    CASE IS <= 30
        pr = 0.02 * ok
    CASE ELSE
        pr = 0.05 * ok
END SELECT
PRINT "premiya="; pr