Aloha is a poor guy who likes riding a bike. But he is too poor to afford a bicycle.
    Recently, the Boom's bike-sharing service entered his school. It only costs 

 yuan for every single rent of a bicycle. From then on, Aloha could rent a bike and enjoy the speed and passion of riding. 
  
 
   The Boom's company has launched a promotion program named the 

. In this program, users can buy discount cards with several free rents and a valid period. For example, after purchasing a discount card with 

 free rents and 

 days of valid time, one doesn't need to pay extra fees for the following 

 rents in the next 

 days. Note that, no matter when the card is bought on day 

, it will always expire at the end of day 

. Also, any new discount card overrides the previous one even if it is still valid, i.e., the remaining free rents of the previous discount card are voided immediately when the new discount card is purchased. 
 There are 

 different types of discount cards in the 

. A card of 

th type with 

 free rents and a valid period of 

 days costs 

 yuan. One can buy any type of discount card unlimited times. 
 Given the rental records, Aloha wants to know the minimum money he has to pay, provided that he takes the best strategy.