(define (digits x)
  (let loop ((n x) (ls '()))
    (if (< n 10)
      (cons n ls)
      (loop (quotient n 10) 
            (cons (modulo n 10) ls)))))

(define (f ls)
  (if (= 1 (length ls))
    (car ls)
    (f (map (lambda (x y) (+  (modulo   (+ x y) 10)
                              (quotient (+ x y) 10)))
            (cdr ls)
            ls))))

(define (input->list)
  (let loop ((line (read)) (ls '()))
    (if (eof-object? line)
      (cdr (reverse ls))
      (loop (read) (cons line ls)))))

(define (MAIN)
  (for-each
    (lambda (ans) (begin (display ans) (newline)))
    (map (lambda (x) (f (digits x))) (input->list))))

(MAIN)