Problem:
Score : 600 points

Problem Statement

Given is a directed graph G with N vertices and M edges.

The vertices are numbered 1 to N, and the i-th edge is directed from Vertex A_i to Vertex B_i.

It is guaranteed that the graph contains no self-loops or multiple edges.

Determine whether there exists an induced subgraph (see Notes) of G such that the in-degree and out-degree of every vertex are both 1. If the answer is yes, show one such subgraph.

Here the null graph is not considered as a subgraph.

Notes

For a directed graph G = (V, E), we call a directed graph G' = (V', E') satisfying the following conditions an induced subgraph of G:

V' is a (non-empty) subset of V.

E' is the set of all the edges in E that have both endpoints in V'.

Constraints

1 \leq N \leq 1000

0 \leq M \leq 2000

1 \leq A_i,B_i \leq N

A_i \neq B_i

All pairs (A_i, B_i) are distinct.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
:
A_M B_M

Output

If there is no induced subgraph of G that satisfies the condition, print -1.
Otherwise, print an induced subgraph of G that satisfies the condition, in the following format:

K
v_1
v_2
:
v_K

This represents the induced subgraph of G with K vertices whose vertex set is \{v_1, v_2, \ldots, v_K\}. (The order of v_1, v_2, \ldots, v_K does not matter.)
If there are multiple subgraphs of G that satisfy the condition, printing any of them is accepted.

Sample Input 1

4 5
1 2
2 3
2 4
4 1
4 3

Sample Output 1

3
1
2
4

The induced subgraph of G whose vertex set is \{1, 2, 4\} has the edge set \{(1, 2), (2, 4), (4, 1)\}. The in-degree and out-degree of every vertex in this graph are both 1.

Sample Input 2

4 5
1 2
2 3
2 4
1 4
4 3

Sample Output 2

-1

There is no induced subgraph of G that satisfies the condition.

Sample Input 3

6 9
1 2
2 3
3 4
4 5
5 6
5 1
5 2
6 1
6 2

Sample Output 3

4
2
3
4
5

Solution in Common Lisp (SBCL 1.1.14) (judge verdict: Accepted)

;; -*- coding: utf-8 -*-
(eval-when (:compile-toplevel :load-toplevel :execute)
  (sb-int:defconstant-eqx OPT
    #+swank '(optimize (speed 3) (safety 2))
    #-swank '(optimize (speed 3) (safety 0) (debug 0))
    #'equal)
  #+swank (ql:quickload '(:cl-debug-print :fiveam) :silent t)
  #-swank (set-dispatch-macro-character
           #\# #\> (lambda (s c p) (declare (ignore c p)) (read s nil nil t))))
#+swank (cl-syntax:use-syntax cl-debug-print:debug-print-syntax)
#-swank (disable-debugger) ; for CS Academy

;; BEGIN_INSERTED_CONTENTS
(declaim (ftype (function * (values fixnum &optional)) read-fixnum))
(defun read-fixnum (&optional (in *standard-input*))
  (declare #.OPT)
  (macrolet ((%read-byte ()
               `(the (unsigned-byte 8)
                     #+swank (char-code (read-char in nil #\Nul))
                     #-swank (sb-impl::ansi-stream-read-byte in nil #.(char-code #\Nul) nil))))
    (let* ((minus nil)
           (result (loop (let ((byte (%read-byte)))
                           (cond ((<= 48 byte 57)
                                  (return (- byte 48)))
                                 ((zerop byte) ; #\Nul
                                  (error "Read EOF or #\Nul."))
                                 ((= byte #.(char-code #\-))
                                  (setf minus t)))))))
      (declare ((integer 0 #.most-positive-fixnum) result))
      (loop
        (let* ((byte (%read-byte)))
          (if (<= 48 byte 57)
              (setq result (+ (- byte 48)
                              (* 10 (the (integer 0 #.(floor most-positive-fixnum 10)) result))))
              (return (if minus (- result) result))))))))

;;;
;;; Binary heap
;;;

(define-condition heap-empty-error (simple-error)
  ((heap :initarg :heap :reader heap-empty-error-heap))
  (:report
   (lambda (condition stream)
     (format stream "Attempted to pop empty heap ~W" (heap-empty-error-heap condition)))))

(define-condition heap-full-error (simple-error)
  ((heap :initarg :heap :reader heap-full-error-heap)
   (item :initarg :item :reader heap-full-error-item))
  (:report
   (lambda (condition stream)
     (format stream "Attempted to push item ~W to full heap ~W"
             (heap-full-error-item condition)
             (heap-full-error-heap condition)))))

(defmacro define-binary-heap (name &key (order '#'>) (element-type 'fixnum))
  "Defines the binary heap specialized for the given order and the element
type. This macro defines a structure of the name NAME and relevant functions:
MAKE-<NAME>, <NAME>-PUSH, <NAME>-POP, <NAME>-REINITIALIZE, <NAME>-EMPTY-P,
<NAME>-COUNT, and <NAME>-PEEK."
  (check-type name symbol)
  (let* ((string-name (string name))
         (fname-push (intern (format nil "~A-PUSH" string-name)))
         (fname-pop (intern (format nil "~A-POP" string-name)))
         (fname-reinitialize (intern (format nil "~A-REINITIALIZE" string-name)))
         (fname-empty-p (intern (format nil "~A-EMPTY-P" string-name)))
         (fname-count (intern (format nil "~A-COUNT" string-name)))
         (fname-peek (intern (format nil "~A-PEEK" string-name)))
         (fname-make (intern (format nil "MAKE-~A" string-name)))
         (acc-position (intern (format nil "~A-POSITION" string-name)))
         (acc-data (intern (format nil "~A-DATA" string-name))))
    `(progn
       (defstruct (,name
                   (:constructor ,fname-make
                       (size
                        &aux (data ,(if (eql element-type '*)
                                        `(make-array (1+ size))
                                        `(make-array (1+ size) :element-type ',element-type))))))
         (data #() :type (simple-array ,element-type (*)) :read-only t)
         (position 1 :type (integer 1 #.most-positive-fixnum)))

       (declaim #+sbcl (sb-ext:maybe-inline ,fname-push))
       (defun ,fname-push (obj heap)
         "Adds OBJ to the end of HEAP."
         (declare (optimize (speed 3))
                  (type ,name heap))
         (symbol-macrolet ((position (,acc-position heap)))
           (let ((data (,acc-data heap)))
             (declare ((simple-array ,element-type (*)) data))
             (labels ((update (pos)
                        (declare (optimize (speed 3) (safety 0)))
                        (unless (= pos 1)
                          (let ((parent-pos (ash pos -1)))
                            (when (funcall ,order (aref data pos) (aref data parent-pos))
                              (rotatef (aref data pos) (aref data parent-pos))
                              (update parent-pos))))))
               (unless (< position (length data))
                 (error 'heap-full-error :heap heap :item obj))
               (setf (aref data position) obj)
               (update position)
               (incf position)
               heap))))

       (declaim #+sbcl (sb-ext:maybe-inline ,fname-pop))
       (defun ,fname-pop (heap)
         "Removes and returns the element at the top of HEAP."
         (declare (optimize (speed 3))
                  (type ,name heap))
         (symbol-macrolet ((position (,acc-position heap)))
           (let ((data (,acc-data heap)))
             (declare ((simple-array ,element-type (*)) data))
             (labels ((update (pos)
                        (declare (optimize (speed 3) (safety 0))
                                 ((integer 1 #.most-positive-fixnum) pos))
                        (let* ((child-pos1 (+ pos pos))
                               (child-pos2 (1+ child-pos1)))
                          (when (<= child-pos1 position)
                            (if (<= child-pos2 position)
                                (if (funcall ,order (aref data child-pos1) (aref data child-pos2))
                                    (unless (funcall ,order (aref data pos) (aref data child-pos1))
                                      (rotatef (aref data pos) (aref data child-pos1))
                                      (update child-pos1))
                                    (unless (funcall ,order (aref data pos) (aref data child-pos2))
                                      (rotatef (aref data pos) (aref data child-pos2))
                                      (update child-pos2)))
                                (unless (funcall ,order (aref data pos) (aref data child-pos1))
                                  (rotatef (aref data pos) (aref data child-pos1))))))))
               (when (= position 1)
                 (error 'heap-empty-error :heap heap))
               (prog1 (aref data 1)
                 (decf position)
                 (setf (aref data 1) (aref data position))
                 (update 1))))))

       (declaim (inline ,fname-reinitialize))
       (defun ,fname-reinitialize (heap)
         "Makes HEAP empty."
         (setf (,acc-position heap) 1)
         heap)

       (declaim (inline ,fname-empty-p))
       (defun ,fname-empty-p (heap)
         "Returns true iff HEAP is empty."
         (= 1 (,acc-position heap)))

       (declaim (inline ,fname-count))
       (defun ,fname-count (heap)
         "Returns the current number of the elements in HEAP."
         (- (,acc-position heap) 1))

       (declaim (inline ,fname-peek))
       (defun ,fname-peek (heap)
         "Returns the topmost element of HEAP."
         (if (= 1 (,acc-position heap))
             (error 'heap-empty-error :heap heap)
             (aref (,acc-data heap) 1))))))

(defmacro dbg (&rest forms)
  #+swank
  (if (= (length forms) 1)
      `(format *error-output* "~A => ~A~%" ',(car forms) ,(car forms))
      `(format *error-output* "~A => ~A~%" ',forms `(,,@forms)))
  #-swank (declare (ignore forms)))

(defmacro define-int-types (&rest bits)
  `(progn
     ,@(mapcar (lambda (b) `(deftype ,(intern (format nil "UINT~A" b)) () '(unsigned-byte ,b))) bits)
     ,@(mapcar (lambda (b) `(deftype ,(intern (format nil "INT~A" b)) () '(signed-byte ,b))) bits)))
(define-int-types 2 4 7 8 15 16 31 32 62 63 64)

(declaim (inline println))
(defun println (obj &optional (stream *standard-output*))
  (let ((*read-default-float-format* 'double-float))
    (prog1 (princ obj stream) (terpri stream))))

(defconstant +mod+ 1000000007)

;;;
;;; Body
;;;

(defconstant +inf+ #xffffffff)
;; dist . v
(define-binary-heap heap
  :order (lambda (x y)
           (< (the uint32 (car x)) (the uint32 (car y))))
  :element-type list)

(defun min-cycle-cost (src dest n graph que)
  (declare ((simple-array list (*)) graph)
           (uint32 n src dest))
  (heap-reinitialize que)
  (let ((dists (make-array (* 2 n) :element-type 'uint32 :initial-element +inf+)))
    (heap-push (cons 0 src) que)
    (loop until (heap-empty-p que)
          for (dist . current) = (heap-pop que)
          when (< dist (aref dists current))
          do (setf (aref dists current) dist)
             (dolist (neighbor (aref graph current))
               (let ((cost (if (= neighbor (+ current n)) 0 1)))
                 (when (< (+ dist cost) (aref dists neighbor))
                   (heap-push (cons (+ dist cost) neighbor) que)))))
    (values (aref dists dest) dists)))

(defun min-cycle (src n graph que)
  (declare ((simple-array list (*)) graph)
           (uint32 n src))
  (heap-reinitialize que)
  (let ((dists (make-array (* 2 n) :element-type 'uint32 :initial-element +inf+))
        (paths (make-array (* 2 n) :element-type 'list :initial-element nil)))
    (heap-push (list 0 src (list src)) que)
    (loop until (heap-empty-p que)
          for (dist current path) = (heap-pop que)
          when (< dist (aref dists current))
          do (setf (aref dists current) dist
                   (aref paths current) path)
             (dolist (neighbor (aref graph current))
               (let ((cost (if (= neighbor (+ current n)) 0 1)))
                 (when (< (+ dist cost) (aref dists neighbor))
                   (heap-push (list (+ dist cost) neighbor (cons neighbor path)) que)))))
    paths))

(defun main ()
  (let* ((n (read))
         (m (read))
         ;; in : out
         (graph (make-array (* 2 n) :element-type 'list :initial-element nil))
         (revgraph (make-array (* 2 n) :element-type 'list :initial-element nil))
         (que (make-heap (+ m n n)))
         (min-value +inf+)
         (min-src 0)
         min-dists)
    (declare (uint32 n m min-value min-src))
    (dotimes (i n)
      (push (+ i n) (aref graph i))
      (push i (aref revgraph (+ i n))))
    (dotimes (i m)
      (let ((a (- (read-fixnum) 1))
            (b (- (read-fixnum) 1)))
        (declare (uint32 a b))
        (push b (aref graph (+ a n)))
        (push (+ a n) (aref revgraph b))))
    (dotimes (i n)
      (let ((src (+ i n))
            (dest i))
        (multiple-value-bind (res dists) (min-cycle-cost src dest n graph que)
          (when (< res min-value)
            (setf min-value res
                  min-src src
                  min-dists dists)))))
    (if (= min-value +inf+)
        (println -1)
        (let* ((paths (min-cycle min-src n graph que))
               (min-dest (- min-src n))
               (path (reverse (aref paths min-dest))))
          (println min-value)
          (dolist (v path)
            (when (< v n)
              (println (+ 1 v))))))))

#-swank (main)