#include<iostream>
using namespace std;

long digital (long n )
{
    int sum  = 0;
    sum = n%10;
    n = n/10;
    while(n !=0)
    {
      sum += n%10;
      n = n/10;      
    }
    if(sum <10)
    return sum;
    else return digital(sum); 
}

int main()
{
    long n = -1;
    
    while(cin>>n&& n!=0)
    {
      cout<< digital(n)<<endl;             
    }
    
}