Problem:
Problem B: Prime Gap

The sequence of n - 1 consecutive composite numbers (positive integers that are not prime and
not equal to 1) lying between two successive prime numbers p and p + n is called a prime gap of length n. For example, (24, 25, 26, 27, 28) between 23 and 29 is a prime gap of length 6.

Your mission is to write a program to calculate, for a given positive integer k, the length of the prime gap that contains k. For convenience, the length is considered 0 in case no prime gap
contains k.

Input

The input is a sequence of lines each of which contains a single positive integer. Each positive
integer is greater than 1 and less than or equal to the 100000th prime number, which is 1299709.
The end of the input is indicated by a line containing a single zero.

Output

The output should be composed of lines each of which contains a single non-negative integer. It
is the length of the prime gap that contains the corresponding positive integer in the input if it
is a composite number, or 0 otherwise. No other characters should occur in the output.

Sample Input

10
11
27
2
492170
0

Output for the Sample Input

4
0
6
0
114

Solution in Haskell (judge verdict: Accepted)

import Control.Applicative
import Control.Monad
import Data.List

main :: IO ()
main = do
  n <- readLn
  unless (n == 0) $ do
    print $ solve n
    main

solve :: Int -> Int
solve n | null ys' = head zs - last xs
        | otherwise = 0
  where (xs, ys) = span (< n) ps
        (ys', zs) = break (> n) ys


isP :: Int -> Bool
isP x = all ((/= 0).(mod x)) . takeWhile ((<= x).(^ 2)) $ ps

ps :: [Int]
ps = 2 : unfoldr f 3
  where f x | isP x = Just (x, x+2)
            | otherwise = f (x+2)