USING: io kernel math math.parser ;
IN: payments

: interest-rate ( -- rate )
    .02 12 / ;

: initial-balance ( -- balance )
    500 ;

: total-to-pay ( -- amt )
    587 ;

: number-of-payments ( -- n )
    6 ;

: monthly-payment ( -- payment )
    total-to-pay number-of-payments / ;

: collect-interest ( rate balance -- rate balance )
    2dup * + ;

: payment ( rate balance payment -- rate balance payment )
    [ collect-interest ] dip [ - ] keep ;

: do-it ( -- )
    interest-rate initial-balance monthly-payment
    number-of-payments [ payment ] times rot 2drop number>string print ;