#include<stdio.h>
int main()
{
	long number,digit;
	while(1)
	{
		scanf("%ld",&number);
		if(number==0)
			break;
		while(1)
		{	
			digit=0;	
			do{
				digit+=number%10;
				number/=10;
			}while(number>0);
			if(digit<10)
				break;
			else 
				number=digit;
		}
		printf("%ld\n",digit);
	}
	return 0;
}