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:
from collections import deque, defaultdict
import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)

N, M = map(int, input().split())
edge = [[] for _ in range(N + 1)]
Redge = [[] for _ in range(N + 1)]
for _ in range(M):
    s, t = map(int, input().split())
    edge[s].append(t)
    Redge[t].append(s)

dp = [0.0] * (N + 1)
for s in reversed(range(1, N)):
    dp[s] = sum(dp[t] + 1.0 for t in edge[s]) / len(edge[s])
ans = dp[1]


cut = set()
for s in range(1, N):
    if len(edge[s]) > 1:
        i = edge[s][0]
        for j in edge[s]:
            if dp[i] < dp[j]:
                i = j
        cut.add((s, i))

for ng in cut:
    dp = [0.0] * (N + 1)
    for s in reversed(range(1, N)):
        E = 0.
        prob = 0.
        for t in edge[s]:
            if (s, t) == ng:
                continue
            prob += 1.0
            E += dp[t] + 1.0
        dp[s] = E / prob
    ans = min(ans, dp[1])

print(ans)