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

stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().rstrip()  # ignore trailing spaces

n, m = na()
g = []
for i in range(n):
    g.append([])
for i in range(m):
    s, t = na()
    g[s-1].append(t-1)

es = [0] * n
for i in range(n-2,-1,-1):
    s = 0
    for e in g[i]:
        s += es[e]
    s = s / len(g[i]) + 1
    es[i] = s

# print(es)

cont = [0] * n
for i in range(n):
    ees = [0] * (i+1)
    ees[-1] = 1.0
    for j in range(i-1,-1,-1):
        t = 0
        for e in g[j]:
            if e <= i:
                t += ees[e]
        ees[j] = t / len(g[j])
    cont[i] = ees[0]
# print(cont)

ans = es[0]
for i in range(n):
    if len(g[i]) == 1:
        continue
    s = 0
    for e in g[i]:
        s += es[e]
    minto = es[i]
    for e in g[i]:
        to = (s - es[e])/(len(g[i])-1) + 1
        minto = min(minto, to)
    ans = min(ans, es[0] - (es[i]-minto)*cont[i])
print("{:.14f}\n".format(ans))