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)

while buf = gets
  primelist, searchlist, numlist, num = [], Array.new(buf.to_i - 1){|i| i + 2}, Array.new(buf.to_i){|i| i + 1}, 0
  unless buf.to_i == 1 || buf.to_i == 2
    loop do
      primelist << searchlist.first
      searchlist.reject! {|num| num % primelist.last == 0}
      break if primelist.last ** 2 > searchlist.last
    end
  end
  numlist.zip(numlist.reverse) {|i, r| num += 1 if (primelist + searchlist).include?(i) && (primelist + searchlist).include?(r)}
  puts num
end