Prime Gap
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Like Fields Medal winner Terrence Tao, you are interested in gaps
between primes.  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

Sample Output
-------------
4
0
6
0
114