Problem:
Pair of Primes

We arrange the numbers between 1 and N (1 <= N <= 10000) in increasing
order and decreasing order like this:

1 2 3 4 5 6 7 8 9 . . . N
N . . . 9 8 7 6 5 4 3 2 1

Two numbers faced each other form a pair. Your task is to compute the number of pairs P such that both
numbers in the pairs are prime.

Input

Input contains several test cases. Each test case consists of an integer N in
one line.

Output

For each line of input, output P .

Sample Input

1
4
7
51

Output for the Sample Input

0
2
2
6

Solution in Ruby (judge verdict: Time Limit Exceeded)

require 'prime'

readlines.each do |l|
  n = l.to_i
  indexes = []
  (1..n).to_a.each_with_index do |x, i|
    indexes.push(i+1) if x.prime?
  end

  count = 0
  indexes.each do |x|
    count += 1 if indexes.include?((n+1)-x)
  end

  puts count
end