All in one 1405: sum and product of prime numbers

【 Title Description 】

The sum of two prime numbers is S, What is the maximum product of them ？

【 Input 】

One is not greater than 10000 The positive integer S, Is the sum of two prime numbers .

【 Output 】

An integer , Is the maximum product of two prime numbers . The data is guaranteed to have a solution .

【 sample input 】

50

【 sample output 】

589

#include<stdio.h>
int main()
{
    int a[10001],b[10001];
    a[0] = 2;
    int i, j;
    int count = 1;
    for (i = 3; i <= 9999; i += 2)
    {
        for (j = 3; j <= i; j += 2)
        {
            if (i % j == 0)
            {
                break;
            }
        }
        if (i == j)
        {
            a[count] = i;
            count++;
        }
    }
    for (i = 0; i < count; i++)
    {
        b[i] = a[i];
    }
    int n;
    scanf("%d", &n);
    int num = 0;
    for (i = 0; i < count; i++)
    {
        for (j = 0; j < count; j++)
        {
            if (a[i] + b[j] == n)
            {
                if (a[i] * b[j] > num)
                {
                    num = a[i] * b[j];
                }
            }
        }
    }
    printf("%d", num);
    return 0;
}