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)

ps = [2, 3]

readlines.each do |l|
  n = l.to_i
  if ps.max < n
    ps.max.upto(n) do |x|
      ps.each do |y|
        break if x % y == 0
        if Math.sqrt(x) < y
          ps.push(x)
          break
        end
      end
    end
  end

  a = (1..n).to_a
  ra = a.reverse
  count = 0
  0.upto(n-1) do |i|
    count += 1 if ps.include?(a[i]) && ps.include?(ra[i])
  end

  puts count
end