Letter Delivery

Postman has to deliver some letters to the famous town $X$ where everyone is helpful and cooperative. The postman has to deliver a total of $N$ letters . The cost of going from the post office to the $i^{th}$ letterbox is given by $c_i (1 \leq i \leq N)$. The postman delivers letter number $i$ and then the receiver takes the letters of all his acquaintances from the postman and delivers them to t