#include<stdio.h>
int main()
{
    int a,sum;
    while(scanf("%d",&a)==1 && a != 0){
        sum = 0;   
        while(a || sum>=10){
            if(a==0 && sum>=10) a=sum,sum=0;
            sum += (a%10);
            a = a/10;
        }
        printf("%d\n",sum);
    }
}