#include <stdio.h>

int main()
{
	int i;
	int num;
	int box = 0;

	scanf("%d", &num);

	//for(i = 0; i < 100; i++)
	{
		box = (num % 10);
		box = box + (num / 10);
		for(i = 0; i < 100; i++)
		{
			box = (box % 10) + (box / 10);
			if((0 <= box) && (box <= 9))
			{
				printf("%d\n", box);
				return 0;
			}
		}
	}

	return 0;
}