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
from collections import deque, defaultdict
import copy
import bisect
sys.setrecursionlimit(10 ** 9)
import math
import heapq
from itertools import product, permutations,combinations
import fractions
from operator import itemgetter


import sys
def input():
	return sys.stdin.readline().strip()

N, M = list(map(int, input().split()))

graph = [[] for _ in range(N)]

for _ in range(M):
	s, t = list(map(int, input().split()))
	graph[s - 1].append(t - 1)

base_dist = [-1]*N
base_dist[-1] = 0


for i in range(N - 2, -1, -1):
	total = 0
	for node in graph[i]:
		total += base_dist[node]
	base_dist[i] = total/len(graph[i]) + 1


ans = base_dist[0]
for stop in range(0, N - 1):
	dist = copy.deepcopy(base_dist)
	total = 0
	if len(graph[stop]) == 1:
		continue
	max_d = 0
	for node in graph[stop]:
		max_d = max(max_d, dist[node])
		total += dist[node]
	total -= max_d
	total /= (len(graph[stop]) - 1)
	total += 1
	dist[stop] = total

	for i in range(stop - 1, -1, -1):
		total = 0
		for node in graph[i]:
			total += dist[node]
		dist[i] = total / len(graph[i]) + 1

	ans = min(ans, dist[0])
	#print(stop,dist)

print(ans)