Problem:
Score : 600 points

Problem Statement

There is a cave consisting of N rooms and M one-directional passages. The rooms are numbered 1 through N.

Takahashi is now in Room 1, and Room N has the exit. The i-th passage connects Room s_i and Room t_i (s_i < t_i) and can only be traversed in the direction from Room s_i to Room t_i. It is known that, for each room except Room N, there is at least one passage going from that room.

Takahashi will escape from the cave. Each time he reaches a room (assume that he has reached Room 1 at the beginning), he will choose a passage uniformly at random from the ones going from that room and take that passage.

Aoki, a friend of Takahashi's, can block one of the passages (or do nothing) before Takahashi leaves Room 1. However, it is not allowed to block a passage so that Takahashi is potentially unable to reach Room N.

Let E be the expected number of passages Takahashi takes before he reaches Room N. Find the value of E when Aoki makes a choice that minimizes E.

Constraints

2 \leq N \leq 600

N-1 \leq M \leq \frac{N(N-1)}{2}

s_i < t_i

If i != j, (s_i, t_i) \neq (s_j, t_j). (Added 21:23 JST)

For every v = 1, 2, ..., N-1, there exists i such that v = s_i.

Input

Input is given from Standard Input in the following format:

N M
s_1 t_1
:
s_M t_M

Output

Print the value of E when Aoki makes a choice that minimizes E.
Your output will be judged as correct when the absolute or relative error from the judge's output is at most 10^{-6}.

Sample Input 1

4 6
1 4
2 3
1 3
1 2
3 4
2 4

Sample Output 1

1.5000000000

If Aoki blocks the passage from Room 1 to Room 2, Takahashi will go along the path 1 → 3 → 4 with probability \frac{1}{2} and 1 → 4 with probability \frac{1}{2}. E = 1.5 here, and this is the minimum possible value of E.

Sample Input 2

3 2
1 2
2 3

Sample Output 2

2.0000000000

Blocking any one passage makes Takahashi unable to reach Room N, so Aoki cannot block a passage.

Sample Input 3

10 33
3 7
5 10
8 9
1 10
4 6
2 5
1 7
6 10
1 4
1 3
8 10
1 5
2 6
6 9
5 6
5 8
3 6
4 8
2 7
2 9
6 7
1 2
5 9
6 8
9 10
3 9
7 8
4 5
2 10
5 7
3 5
4 7
4 9

Sample Output 3

3.0133333333

Solution in Common Lisp (SBCL 1.1.14) (judge verdict: Wrong Answer)

#-swank
(unless (member :child-sbcl *features*)
  (quit
   :unix-status
   (process-exit-code
    (run-program *runtime-pathname*
                 `("--control-stack-size" "128MB"
                   "--noinform" "--disable-ldb" "--lose-on-corruption" "--end-runtime-options"
                   "--eval" "(push :child-sbcl *features*)"
                   "--script" ,(namestring *load-pathname*))
                 :output t :error t :input t))))
;; -*- 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
;;;
;;; Memoization macro
;;;

;;
;; Basic usage:
;;
;; (with-cache (:hash-table :test #'equal :key #'cons)
;;   (defun add (a b)
;;     (+ a b)))
;; This function caches the returned values for already passed combinations of
;; arguments. In this case ADD stores the key (CONS A B) and the return value to
;; a hash-table when evaluating (ADD A B) for the first time. ADD returns the
;; stored value when it is called with the same arguments (w.r.t. EQUAL) again.
;;
;; The storage for the cache is hash-table or array. Let's see an example for
;; array:
;; (with-cache (:array (10 20 30) :initial-element -1 :element-type 'fixnum)
;;   (defun foo (a b c) ... ))
;; This form stores the value of FOO in the array created by (make-array (list
;; 10 20 30) :initial-element -1 :element-type 'fixnum). Note that
;; INITIAL-ELEMENT must always be given here as it is used as the flag for `not
;; yet stored'. (Therefore INITIAL-ELEMENT should be a value FOO never takes.)
;;
;; If you want to ignore some arguments, you can put `*' in dimensions:
;; (with-cache (:array (10 10 * 10) :initial-element -1)
;;   (defun foo (a b c d) ...)) ; then C is ignored when querying or storing cache
;;
;; Available definition forms in WITH-CACHE are DEFUN, LABELS, FLET, and
;; SB-INT:NAMED-LET.
;;
;; You can trace the memoized function by :TRACE option:
;; (with-cache (:array (10 10) :initial-element -1 :trace t)
;;   (defun foo (x y) ...))
;; Then FOO is traced as with CL:TRACE.
;;

;; TODO & NOTE: Currently a memoized function is not enclosed with a block of
;; the function name.

;; FIXME: *RECURSION-DEPTH* should be included within the macro.
(declaim (type (integer 0 #.most-positive-fixnum) *recursion-depth*))
(defparameter *recursion-depth* 0)

(eval-when (:compile-toplevel :load-toplevel :execute)
  (defun %enclose-with-trace (fname args form)
    (let ((value (gensym)))
      `(progn
         (format t "~&~A~A: (~A ~{~A~^ ~}) =>"
                 (make-string *recursion-depth*
                              :element-type 'base-char
                              :initial-element #\ )
                 *recursion-depth*
                 ',fname
                 (list ,@args))
         (let ((,value (let ((*recursion-depth* (1+ *recursion-depth*)))
                         ,form)))
           (format t "~&~A~A: (~A ~{~A~^ ~}) => ~A"
                   (make-string *recursion-depth*
                                :element-type 'base-char
                                :initial-element #\ )
                   *recursion-depth*
                   ',fname
                   (list ,@args)
                   ,value)
           ,value))))

  (defun %extract-declarations (body)
    (remove-if-not (lambda (form) (and (consp form) (eql 'declare (car form))))
                   body))

  (defun %parse-cache-form (cache-specifier)
    (let ((cache-type (car cache-specifier))
          (cache-attribs (cdr cache-specifier)))
      (assert (member cache-type '(:hash-table :array)))
      (let* ((dims-with-* (when (eql cache-type :array) (first cache-attribs)))
             (dims (remove '* dims-with-*))
             (rank (length dims))
             (rest-attribs (ecase cache-type
                             (:hash-table cache-attribs)
                             (:array (cdr cache-attribs))))
             (key (prog1 (getf rest-attribs :key) (remf rest-attribs :key)))
             (trace-p (prog1 (getf rest-attribs :trace) (remf rest-attribs :trace)))
             (cache-form (case cache-type
                           (:hash-table `(make-hash-table ,@rest-attribs))
                           (:array `(make-array (list ,@dims) ,@rest-attribs))))
             (initial-element (when (eql cache-type :array)
                                (assert (member :initial-element rest-attribs))
                                (getf rest-attribs :initial-element))))
        (let ((cache (gensym "CACHE"))
              (value (gensym))
	      (present-p (gensym))
              (name-alias (gensym))
	      (args-lst (gensym))
              (indices (loop repeat rank collect (gensym))))
          (labels
              ((make-cache-querier (cache-type name args)
                 (let ((res (case cache-type
                              (:hash-table
                               `(let ((,args-lst (funcall ,(or key #'list) ,@args)))
                                  (multiple-value-bind (,value ,present-p)
                                      (gethash ,args-lst ,cache)
                                    (if ,present-p
                                        ,value
                                        (setf (gethash ,args-lst ,cache)
                                              (,name-alias ,@args))))))
                              (:array
                               (let ((memoized-args (loop for dimension in dims-with-*
                                                          for arg in args
                                                          unless (eql dimension '*)
                                                          collect arg)))
                                 (if key
                                     `(multiple-value-bind ,indices
                                          (funcall ,key ,@memoized-args)
                                        (let ((,value (aref ,cache ,@indices)))
                                          (if (eql ,initial-element ,value)
                                              (setf (aref ,cache ,@indices)
                                                    (,name-alias ,@args))
                                              ,value)))
                                     `(let ((,value (aref ,cache ,@memoized-args)))
                                        (if (eql ,initial-element ,value)
                                            (setf (aref ,cache ,@memoized-args)
                                                  (,name-alias ,@args))
                                            ,value))))))))
                   (if trace-p
                       (%enclose-with-trace name args res)
                       res)))
               (make-reset-form (cache-type)
                 (case cache-type
                   (:hash-table `(setf ,cache (make-hash-table ,@rest-attribs)))
                   (:array `(prog1 nil
                              (fill (array-storage-vector ,cache) ,initial-element)))))
               (make-reset-name (name)
                 (intern (format nil "RESET-~A" (symbol-name name)))))
            (values cache cache-form cache-type name-alias
                    #'make-reset-name
                    #'make-reset-form
                    #'make-cache-querier)))))))

(defmacro with-cache ((cache-type &rest cache-attribs) def-form)
  "CACHE-TYPE := :HASH-TABLE | :ARRAY.
DEF-FORM := definition form with DEFUN, LABELS, FLET, or SB-INT:NAMED-LET."
  (multiple-value-bind (cache-symbol cache-form cache-type name-alias
                        make-reset-name make-reset-form
                        make-cache-querier)
      (%parse-cache-form (cons cache-type cache-attribs))
    (ecase (car def-form)
      ((defun)
       (destructuring-bind (_ name args &body body) def-form
         (declare (ignore _))
         `(let ((,cache-symbol ,cache-form))
            (defun ,(funcall make-reset-name name) ()
              ,(funcall make-reset-form cache-type))
            (defun ,name ,args
              ,@(%extract-declarations body)
              (labels ((,name-alias ,args ,@body))
                (declare (inline ,name-alias))
                ,(funcall make-cache-querier cache-type name args))))))
      ((labels flet)
       (destructuring-bind (_ definitions &body labels-body) def-form
         (declare (ignore _))
         (destructuring-bind (name args &body body) (car definitions)
           `(let ((,cache-symbol ,cache-form))
              (,(car def-form)
               ((,(funcall make-reset-name name) ()
                 ,(funcall make-reset-form cache-type))
                (,name ,args
                       ,@(%extract-declarations body)
                       (labels ((,name-alias ,args ,@body))
                         (declare (inline ,name-alias))
                         ,(funcall make-cache-querier cache-type name args)))
                ,@(cdr definitions))
               (declare (ignorable #',(funcall make-reset-name name)))
               ,@labels-body)))))
      ((nlet #+sbcl sb-int:named-let)
       (destructuring-bind (_ name bindings &body body) def-form
         (declare (ignore _))
         `(let ((,cache-symbol ,cache-form))
            (,(car def-form) ,name ,bindings
             ,@(%extract-declarations body)
             ,(let ((args (mapcar (lambda (x) (if (atom x) x (car x))) bindings)))
                `(labels ((,name-alias ,args ,@body))
                   (declare (inline ,name-alias))
                   ,(funcall make-cache-querier cache-type name args))))))))))

(defmacro with-caches (cache-specs def-form)
  "DEF-FORM := definition form by LABELS or FLET.

 (with-caches (cache-spec1 cache-spec2)
   (labels ((f (x) ...) (g (y) ...))))
is equivalent to the line up of
 (with-cache cache-spec1 (labels ((f (x) ...))))
and
 (with-cache cache-spec2 (labels ((g (y) ...))))

This macro will be useful to do mutual recursion between memoized local
functions."
  (assert (member (car def-form) '(labels flet)))
  (let (cache-symbol-list cache-form-list cache-type-list name-alias-list make-reset-name-list make-reset-form-list make-cache-querier-list)
    (dolist (cache-spec (reverse cache-specs))
      (multiple-value-bind (cache-symbol cache-form cache-type name-alias
                            make-reset-name make-reset-form make-cache-querier)
          (%parse-cache-form cache-spec)
        (push cache-symbol cache-symbol-list)
        (push cache-form cache-form-list)
        (push cache-type cache-type-list)
        (push name-alias name-alias-list)
        (push make-reset-name make-reset-name-list)
        (push make-reset-form make-reset-form-list)
        (push make-cache-querier make-cache-querier-list)))
    (labels ((def-name (def) (first def))
             (def-args (def) (second def))
             (def-body (def) (cddr def)))
      (destructuring-bind (_ definitions &body labels-body) def-form
        (declare (ignore _))
        `(let ,(loop for cache-symbol in cache-symbol-list
                     for cache-form in cache-form-list
                     collect `(,cache-symbol ,cache-form))
           (,(car def-form)
            (,@(loop for def in definitions
                     for cache-type in cache-type-list
                     for make-reset-name in make-reset-name-list
                     for make-reset-form in make-reset-form-list
                     collect `(,(funcall make-reset-name (def-name def)) ()
                               ,(funcall make-reset-form cache-type)))
             ,@(loop for def in definitions
                     for cache-type in cache-type-list
                     for name-alias in name-alias-list
                     for make-cache-querier in make-cache-querier-list
                     collect `(,(def-name def) ,(def-args def)
                               ,@(%extract-declarations (def-body def))
                               (labels ((,name-alias ,(def-args def) ,@(def-body def)))
                                 (declare (inline ,name-alias))
                                 ,(funcall make-cache-querier cache-type (def-name def) (def-args def))))))
            (declare (ignorable ,@(loop for def in definitions
                                        for make-reset-name in make-reset-name-list
                                        collect `#',(funcall make-reset-name
                                                             (def-name def)))))
            ,@labels-body))))))

(declaim (ftype (function * (values fixnum &optional)) read-fixnum))
(defun read-fixnum (&optional (in *standard-input*))
  (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))))))))

(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
;;;

;; Reference: http://drken1215.hatenablog.com/entry/2019/03/20/202800
(defun echelon! (matrix &optional extended)
  "Returns the row echelon form of MATRIX by gaussian elimination and returns
the rank as the second value.

This function destructively modifies MATRIX."
  (labels ((%zerop (x)
             (< (abs x) 1d-9)))
    (destructuring-bind (m n) (array-dimensions matrix)
      (declare ((integer 0 #.most-positive-fixnum) m n))
      (let ((rank 0))
        (dotimes (target-col (if extended (- n 1) n))
          (let ((pivot-row (do ((i rank (+ 1 i)))
                               ((= i m) -1)
                             (unless (%zerop (aref matrix i target-col))
                               (return i)))))
            (when (>= pivot-row 0)
              ;; swap rows
              (loop for j from target-col below n
                    do (rotatef (aref matrix rank j) (aref matrix pivot-row j)))
              (let ((inv (/ (aref matrix rank target-col))))
                (dotimes (j n)
                  (setf (aref matrix rank j)
                        (* inv (aref matrix rank j))))
                (dotimes (i m)
                  (unless (or (= i rank) (%zerop (aref matrix i target-col)))
                    (let ((factor (aref matrix i target-col)))
                      (loop for j from target-col below n
                            do (setf (aref matrix i j)
                                     (- (aref matrix i j)
                                        (* (aref matrix rank j) factor))))))))
              (incf rank))))
        (values matrix rank)))))

(defun solve-linear-system (matrix vector)
  "Solves Ax ≡ b and returns a root vector if it exists. Otherwise it returns
NIL. In addition, this function returns the rank of A as the second value."
  (destructuring-bind (m n) (array-dimensions matrix)
    (declare ((integer 0 #.most-positive-fixnum) m n))
    (assert (= n (length vector)))
    (let ((extended (make-array (list m (+ n 1)) :element-type (array-element-type matrix))))
      (labels ((%zerop (x)
                 (< (abs x) 1d-9)))
        (dotimes (i m)
          (dotimes (j n) (setf (aref extended i j) (aref matrix i j)))
          (setf (aref extended i n) (aref vector i)))
        (let ((rank (nth-value 1 (echelon! extended t))))
          (if (loop for i from rank below m
                    always (%zerop (aref extended i n)))
              (let ((result (make-array m
                                        :element-type (array-element-type matrix)
                                        :initial-element 0)))
                (dotimes (i rank)
                  (setf (aref result i) (aref extended i n)))
                (values result rank))
              (values nil rank)))))))

;; (defun main ()
;;   (let* ((n (read))
;;          (m (read))
;;          (graph (make-array n :element-type 'list :initial-element nil))
;;          (revgraph (make-array n :element-type 'list :initial-element nil))
;;          (out-degrees (make-array n :element-type 'uint31))
;;          (in-degrees (make-array n :element-type 'uint31))
;;          (mat (make-array (list n n) :element-type 'double-float :initial-element 0d0)))
;;     (dotimes (i m)
;;       (let ((src (- (read-fixnum) 1))
;;             (dest (- (read-fixnum) 1)))
;;         (push dest (aref graph src))
;;         (push src (aref revgraph dest))
;;         (incf (aref out-degrees src))
;;         (incf (aref in-degrees dest))))
;;     (dotimes (w n)
;;       (let ((adj-list (aref revgraph w)))
;;         (dolist (v adj-list)
;;           (setf (aref mat w v)
;;                 (/ (float (aref out-degrees v) 1d0))))))
;;     #>mat
;;     (dotimes (i n)
;;       (dotimes (j n)
;;         (if (= i j)
;;             (setf (aref mat i j) (- 1d0 (aref mat i j)))
;;             (setf (aref mat i j) (- (aref mat i j))))))
;;     mat))

(defun main ()
  (let* ((n (read))
         (m (read))
         (graph (make-array n :element-type 'list :initial-element nil))
         (revgraph (make-array n :element-type 'list :initial-element nil))
         (out-degrees (make-array n :element-type 'uint31))
         (in-degrees (make-array n :element-type 'uint31))
         (ss (make-array m :element-type 'uint31))
         (ts (make-array m :element-type 'uint31)))
    (dotimes (i m)
      (let ((src (- (read-fixnum) 1))
            (dest (- (read-fixnum) 1)))
        (push dest (aref graph src))
        (push src (aref revgraph dest))
        (incf (aref out-degrees src))
        (incf (aref in-degrees dest))
        (setf (aref ss i) src)
        (setf (aref ts i) dest)))
    #>out-degrees
    (with-cache (:array (601 (+ m 1)) :element-type 'double-float :initial-element most-negative-double-float)
      (labels ((calc (v seg-id)
                 (if (zerop v)
                     0d0
                     (let ((exists nil)
                           (value 0d0)
                           (src (if (= seg-id m) -1 (aref ss seg-id)))
                           (dest (if (= seg-id m) -1 (aref ts seg-id))))
                       (dolist (prev (aref revgraph v))
                         (unless (and (= prev src) (= dest v))
                           (let ((delta (calc prev seg-id)))
                             (unless (= delta most-positive-double-float)
                               (setq exists t)
                               (incf value (/ delta (aref out-degrees prev)))))))
                       (if exists
                           (+ value 1d0)
                           most-positive-double-float)))))
        (let ((res (calc (- n 1) m)))
          (loop for seg-id from 0 below m
                do (let ((src (aref ss seg-id))
                         (dest  (aref ts seg-id)))
                     (decf (aref out-degrees src))
                     #>out-degrees
                     (let ((value #>(calc (- n 1) seg-id)))
                       (unless (= most-positive-double-float value)
                         (setq res (min res value))))
                     (incf (aref out-degrees src))))
          (println res))))))

#-swank (main)

;;;
;;; Test and benchmark
;;;

#+swank
(defun io-equal (in-string out-string &key (function #'main) (test #'equal))
  "Passes IN-STRING to *STANDARD-INPUT*, executes FUNCTION, and returns true if
the string output to *STANDARD-OUTPUT* is equal to OUT-STRING."
  (labels ((ensure-last-lf (s)
             (if (eql (uiop:last-char s) #\Linefeed)
                 s
                 (uiop:strcat s uiop:+lf+))))
    (funcall test
             (ensure-last-lf out-string)
             (with-output-to-string (out)
               (let ((*standard-output* out))
                 (with-input-from-string (*standard-input* (ensure-last-lf in-string))
                   (funcall function)))))))

#+swank
(defun get-clipbrd ()
  (with-output-to-string (out)
    (run-program "C:/msys64/usr/bin/cat.exe" '("/dev/clipboard") :output out)))

#+swank (defparameter *this-pathname* (uiop:current-lisp-file-pathname))
#+swank (defparameter *dat-pathname* (uiop:merge-pathnames* "test.dat" *this-pathname*))

#+swank
(defun run (&optional thing (out *standard-output*))
  "THING := null | string | symbol | pathname

null: run #'MAIN using the text on clipboard as input.
string: run #'MAIN using the string as input.
symbol: alias of FIVEAM:RUN!.
pathname: run #'MAIN using the text file as input."
  (let ((*standard-output* out))
    (etypecase thing
      (null
       (with-input-from-string (*standard-input* (delete #\Return (get-clipbrd)))
         (main)))
      (string
       (with-input-from-string (*standard-input* (delete #\Return thing))
         (main)))
      (symbol (5am:run! thing))
      (pathname
       (with-open-file (*standard-input* thing)
         (main))))))

#+swank
(defun gen-dat ()
  (uiop:with-output-file (out *dat-pathname* :if-exists :supersede)
    (format out "")))

#+swank
(defun bench (&optional (out (make-broadcast-stream)))
  (time (run *dat-pathname* out)))

;; To run: (5am:run! :sample)
#+swank
(it.bese.fiveam:test :sample
  (it.bese.fiveam:is
   (common-lisp-user::io-equal "4 6
1 4
2 3
1 3
1 2
3 4
2 4
"
    "1.5000000000
"))
  (it.bese.fiveam:is
   (common-lisp-user::io-equal "3 2
1 2
2 3
"
    "2.0000000000
"))
  (it.bese.fiveam:is
   (common-lisp-user::io-equal "10 33
3 7
5 10
8 9
1 10
4 6
2 5
1 7
6 10
1 4
1 3
8 10
1 5
2 6
6 9
5 6
5 8
3 6
4 8
2 7
2 9
6 7
1 2
5 9
6 8
9 10
3 9
7 8
4 5
2 10
5 7
3 5
4 7
4 9
"
    "3.0133333333
")))