stacking boxes

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <stack>
using namespace std;
int isPrime(int n) {
    int i;
    if(n == 2) return 1;
    if(n%2 == 0) return 0;
    for(i = 3; i*i<=n; i+=2) {
        if(n%i == 0) return 0;
    }
    return 1;
}
int main() {
    int a;
        stack<int>s1;
        stack<int>s2;
        while(cin>>a){
                if(isPrime(a)){
                        s1.push(a);

                }
                else{
                        s2.push(a);
                }
                if(a == 2 || a == 4){
                        break;
                }
        }
        if(s1.top()== 2){
                while(!s2.empty()){
                       cout<<s2.top()<<endl;
                        s2.pop();
                }
                while(!s1.empty()){
                      cout<<s1.top()<<endl;
                     s1.pop();
                }
        }
        else if(s2.top()==4){
                while(!s1.empty()){
                      cout<<s1.top()<<endl;
                     s1.pop();
                }
                while(!s2.empty()){
                       cout<<s2.top()<<endl;
                        s2.pop();
                }
        }
    return 0;
}