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)

# encoding: utf-8                                                            

require 'prime'

while n = gets and (not n.nil?)
  n = n.to_i
  ans = 0
  (1..n).each do |i|   
    ans += 1 if Prime.prime?(i) and Prime.prime?(n - i + 1)
  end
  p ans
end