Problem: 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}.

Examples

Input

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


Output

1.5000000000


Input

3 2
1 2
2 3


Output

2.0000000000


Input

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


Output

3.0133333333

Solution (in PYTHON3):

Solution:
n, m = map(int, input().split())
G = [[] for _ in range(n)]
ST = tuple(tuple(map(lambda x:int(x)-1, input().split())) for _ in range(m))
for s, t in ST:
  G[s].append(t)
#枝(s, t)を塞いだ時にそれぞれそこからあと何手かかるかを求める
def f(s, t):
  dp = [0]*n #そこからあと何手かかるかの期待値
  for i in range(n-2, -1, -1):
    temp = 0
    for v in G[i]:
      temp += dp[v]
    if i==s:
      temp -= dp[t]
      dp[i] = temp/(len(G[i])-1) + 1
    else:
      dp[i] = temp/len(G[i]) + 1
  return dp
T = f(-1, -1)
ans = T[0]
for i in range(n-1):
  if len(G[i]) == 1:
    continue
  m = 0
  t = -1
  for v in G[i]:
    if T[v] > m:
      m = T[v]
      t = v
  temp = f(i, t)[0]
  ans = min(ans, temp)
print(ans)