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'

def main
  while n = gets
    n = n.chomp.to_i
    puts solve(n)
  end
end

def solve n
  res = 0
  (1..n).each do |i|
    o = n - i + 1
    res += 1 if Prime.prime? i and Prime.prime? o
  end
  res
end

main