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 CPP):

Solution:
// F.

#include <iostream>
#include <sstream>
#include <algorithm>
#include <vector>

using namespace std;

int main(int argc, char* argv[]) {
	cout.precision(10);
#ifdef _MSC_VER
	while (true)
#endif
	{
		int n = -1, m = -1;
		cin >> n >> m;
		if (n <= 0) return 0;
		vector<vector<int>> fwd(n);
		vector<double> dp(n, 0);
		for (int i = 0; i < m; ++i) {
			int a, b;
			cin >> a >> b;
			fwd[a - 1].push_back(b - 1);
		}
		double ans = 1e9;
		for (int i = 0; i < n - 1; ++i) {
			for (int j = n - 2; j >= 0; --j) {
				double sum = 0, mx = 0;
				for (int next : fwd[j]) {
					double d = dp[next] + 1;
					sum += d;
					mx = max(mx, d);
				}
				if (i == j && fwd[j].size() >= 2) {
					dp[j] = (sum - mx) / (fwd[j].size() - 1);
				} else {
					dp[j] = sum / fwd[j].size();
				}
			}
			ans = min(ans, dp[0]);
		}
		cout << ans << endl;
	}
	return 0;
}