关于质数:https://blog.csdn.net/qq_41045071/article/details/82762126
#include<stdio.h>
#include<math.h>
int f(int num)
{
if (num <= 3) {
return num > 1;
}
// 不在6的倍数两侧的一定不是质数
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;
}