construire cifre prime

#include<iostream>
using namespace std;
int main()
{
    int n,x,r,p;
    cin>>n;
    p=1;
    r=0;
    do
    {
        x=n%10;
        if(x==3||x==7||x==2||x==1)
        {
            r=r+x*p;
            p=p*10;
        }
        n=n/10;
    }while(n!=0);
    cout<<r;
    return 0;
}