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),*d = [list(map(int, s.split())) for s in open(0)]

d.sort()
for i in range(m):
    d[i] = [d[i][0]-1, d[i][1]-1]

l = [0] * n
for x in d:
    l[x[0]] += 1

p = [0] * n
p[0] = 1
for x in d:
    p[x[1]] += p[x[0]]/l[x[0]]

e = [0] * n
index = d[-1][0]
max_in_index = 0
max_reduce_e = 0

for x in d[::-1] + [[-1, -1]]:
    if index != x[0]:
        if l[index] > 1:
            max_reduce_e = max(max_reduce_e, 
                               p[index] * (e[index]/l[index] - (e[index] - max_in_index)/(l[index] - 1)))
        e[index] /= l[index]
        index = x[0]
        max_in_index = 0
        if index == -1:
            break
    max_in_index = max(max_in_index, e[x[1]] + 1)
    e[x[0]] += e[x[1]] + 1

print(e[0] - max_reduce_e)