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:
#"""
import sys
input = sys.stdin.readline
#"""

N,M = map(int,input().split())
ST = [list(map(int,input().split())) for _ in [0]*M]

Ef = [[] for _ in [0]*N]
Eb = [[] for _ in [0]*N]

for s,t in ST:
    s -= 1
    t -= 1
    Ef[s].append(t)
    Eb[t].append(s)

V = [0]*N
for i in range(N-2,-1,-1):
    tmp_sum = 0
    tmp_num = 0
    for j in Ef[i]:
        tmp_num += 1
        tmp_sum += V[j]
    V[i] = tmp_sum/tmp_num + 1.0

bq = []
for bi in range(i,N-1):
    if len(Ef[bi]) <= 1:
        continue
    bj = 0
    tmp = 0
    for j in Ef[bi]:
        if tmp<V[j]:
            tmp = V[j]
            bj = j
    bq.append((bi,bj))

ans = V[0]
for bi,bj in bq:
    V2 = [0]*N
    for i in range(N-2,-1,-1):
        tmp_sum = 0
        tmp_num = 0
        for j in Ef[i]:
            if i==bi and j==bj : continue
            tmp_num += 1
            tmp_sum += V2[j]
        V2[i] = tmp_sum/tmp_num + 1.0
    ans = min(ans,V2[0])
print(ans)