Cifra de control

#include <iostream>
using namespace std;

int main()
{
   int n, s=0, c;
   cout<<"n="; cin>>n;
   while(n>9)
   {
       s=0;
       while(n!=0)
        {
        c=n%10;
        s=s+c;
        n=n/10;
        }
        n=s;
   }

   cout<<s;
    return 0;
}