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 C# (judge verdict: Accepted)

using System;

public class p1004
{
	const int mx = 10001;
	
	p1004 ()
	{
		string str;
		int n, cnt;
		bool[] prime = new bool[mx];
		
		for (int i = 0; i < mx; ++i)
			prime [i] = true;
		prime [0] = prime [1] = false;
		
		for (int i = 2; i < mx; ++i)
			if (prime [i])
				for (int j = i+i; j < mx; j+=i)
					prime [j] = false;
		
		while ((str=Console.ReadLine())!= null) {
			n = int.Parse (str);
			cnt = 0;
			for (int i = 1, j = n; i <= n && j >= 1; ++i, --j)
				if (prime [i] && prime [j])
					++cnt;
			Console.WriteLine ("{0}", cnt);
		}
	}
	
	public static int Main (string[] args)
	{
		new p1004 ();
		return 0;
	}
}