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 Ruby (2.3.3) (judge verdict: Accepted)

def min(a,b); a < b ? a : b; end
def max(a,b); a > b ? a : b; end

N, M = gets.split.map(&:to_i)
G = Array.new(N + 1){ [] }

Q = Array.new(N + 1, 0.0); Q[1] = 1.0
dp = 2.times.map{ Array.new(N + 1, 0.0) } 

M.times do
  s,t = gets.split.map(&:to_i)
  G[s] << t
end

(1 ... N).reverse_each do |s|
  dmax = 0.0
  sum = 0.0
  
  G[s].each do |t| 
    sum += dp[0][t]
    dmax = max(dp[0][t], dmax)
  end

  dp[0][s] = sum / G[s].size + 1.0

  if G[s].size == 1
    dp[1][s] = dp[0][s]
  else
    dp[1][s] = (sum - dmax) / (G[s].size - 1) + 1.0
  end
end

(1 ... N).each do |s|
  q = Q[s] / G[s].size
  G[s].each{|t| Q[t] += q }
end

ans = (1 ... N).inject(dp[0][1]) do |d_, s|
  min(d_, dp[0][1] + Q[s] * (dp[1][s] - dp[0][s]))
end

puts ans