#include<stdio.h>
#include<math.h> 
int f(int num)
{
    if (num <= 3) {
        return num > 1;
    }
    // 不在6的倍數兩側的一定不是質數
    if (num % 6 != 1 && num % 6 != 5) {
        return 0;
    }
    int n = (int) sqrt((double)num);
    for (int i = 5; i <= n; i += 6) {
        if (num % i == 0 || num % (i + 2) == 0) {
            return 0;
        }
    }
    return 1;
}
int main()
{
	int S;
	scanf("%d",&S);
	int n=S/2;
	int m=n+S%2;
	while(n&&m)
	{
		if(f(n)&&f(m))
			{printf("%d\n",n*m);return 0;}
		n--;
		m++;
	}
	return 0;
}