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: Accepted)

### constant

MAX_P = 10000

### subroutines

def gen_primes(max_p)
  primes = Array.new(max_p + 1){true}
  primes[0] = primes[1] = false

  s_max_p = Math.sqrt(max_p + 0.5).to_i

  for p in (2..s_max_p)
    if primes[p]
      for q in (p * p).step(max_p, p)
        primes[q] = false
      end
    end
  end

  primes
end

### main

primes = gen_primes(MAX_P)

while line = gets
  n = line.strip.to_i

  count = 0

  for i in (1..n)
    if primes[i] && primes[n + 1 - i]
      count += 1
    end
  end

  puts count
end