Hi guys,

Here's a problem from the 2016 BIO round 1. This post is not specifically about the problem but the restring on l.

A prime number is a whole number, greater than 1, that can only be divided by itself and the number 1.
Two prime numbers are connected if the difference between them is 2n for some whole number n ≥ 0;
e.g. possible differences are 1, 2, 4, 8, 16, 32, …
A path is a sequence of (at least two) prime numbers, without repetition, where adjacent numbers in the
sequence are connected. If the first number in the sequence is p and the last number is q then we say
the path is between p and q.
The length of a path is the total number of prime numbers used. There may be multiple paths between
two prime numbers; the lengths of these paths may be different.
For example:
• 13 is connected to 5 (13 — 5 = 8 = 23), 5 is connected to 3 (5 — 3 = 2 = 21) and 3 is connected to 2
(3 — 2 = 1 = 20);
• As 13 and 5 are connected there is a path between them (13—5) whose length is 2;
• There is a path from 13 to 2 (13—5—3—2) whose length is 4;
• There is a longer path from 13 to 2 (13—17—19—3—2) whose length is 5.
You will be given an upper limit on the primes you are allowed to use. For example, if the limit was 18
then the path 13—17—19—3—2 would not be permitted as it includes a prime above this limit.