初学者，从书上抄的！可以运行，但为什么得不到想要的结果？？

题目：从键盘上输入一个大于等于6的偶数，将它表示成两个素数之和。如：6=3+2+1,8=3+5。然后我输入18,24的时候没反应，只有num=3+（另一个素数）   才能显示出来。。
#include <math.h>
#include <stdio.h>
#include <iostream>
using namespace std;
void main()
{
    int i,j,num;
    int p,q,flagp,flagq;
    std::cout<<"please input a plus integer:"<<endl;
    std::cin>>num;
    if(num%2!=0||num<=4)
        std::cout<<"dota error!please try again!"<<endl;
    else
    {
        p=1;
        do{
            p=p+1;
            q=num-p;
            flagp=1;
            flagq=1;
            
            for(i=2;i<=(int)(floor(sqrt((double)(p))));i++)
            {
                if(p%i==0);
                {
                    flagp=0;
                    break;
                }
            }
            j=2;
            while(j<=(int)(floor(sqrt((double)(q)))))
            {
                if(q%j==0)
                {
                    flagq=0;
                    break;
                }
                j++;
            }
        }
            while(flagp*flagq==0);
            std::cout<<num<<"="<<p<<"+"<<q<<endl;
    }


}