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)

def is_prime(x)
  for i in (2...x)
    return false if x % i == 0
    return true if i * i > x
  end
end

$isp = Array.new(2, false)
for i in (2..10000)
  $isp.push(is_prime(i))
end

def count(n)
  ans = 0
  (1..n).each do |i|
    ans += 1 if $isp[i] and $isp[n + 1 - i]
  end
  ans
end

while line = gets
  puts count(line.to_i)
end