啊啊啊为什么会WA？？？

求调

#include<bits/stdc++.h>
using namespace std;
#define int long long

namespace FastIO
{
	const int Mt = 1e5;
	inline char getch()
	{
		static char buf[Mt],*p1 = buf,*p2 = buf;
		return p1 == p2 && (p2 = (p1 = buf) + fread(buf,1,Mt,stdin),p1 == p2) ? EOF : *p1++;
	}
	
	inline int input()
	{
		int num = 0,f = 1;
		char ch = getch();
		while(ch < '0' || ch > '9')
		{
			if(ch == '-') f = -1;
			ch = getch();
		}
		while(ch >= '0' && ch <= '9')
		{
			num = (num << 1) + (num << 3) + (ch ^ 48);
			ch = getch();
		}
		return num * f;
	}
	
	inline void printNum(int num)
	{
		if(num >= 10) printNum(num / 10);
		putchar(num % 10 + 48);
	}
	
	inline void print(int num,char ch = ' ')
	{
		if(num < 0) putchar('-'),num = -num;
		printNum(num);
		putchar(ch);
	}
}
using FastIO::input;
using FastIO::print;

const int N = 1e6 + 5;
const int INF = -1u / 2;
int n,m,cnt,top,timeStamp;
double ans;
int dfn[N],low[N],st[N],ext[N],din[N],bel[N],dout[N];
bool cst[N];
vector <int> edge[N];

void tarjan(int u)
{
	dfn[u] = low[u] = ++timeStamp;
	st[++top] = u,cst[u] = true;
	
	for(int v : edge[u])
	{
		if(!dfn[v])
		{
			tarjan(v);
			low[u] = min(low[u],low[v]);
		}
		else if(cst[v]) low[u] = min(low[u],dfn[v]);
	}
	
	int tmp = 0;
	if(low[u] == dfn[u])
	{
		cnt++;
		do tmp = st[top--],cst[tmp] = false,
		bel[tmp] = cnt,ext[cnt]++;
		while(tmp != u);
	}
}

void create(int u)//求入度出度
{
	for(int v : edge[u])
		if(bel[u] != bel[v])
			din[bel[v]]++,dout[bel[u]]++;
}

signed main()
{
	ans = 0;
	n = input(),m = input();
	// if(n == 1) return puts("1.000000") & 0;
	for(int i = 1;i <= m;i++)
	{
		int u = input(),v = input();
		edge[u].push_back(v);
	}
	
	for(int i = 1;i <= n;i++)
		if(!dfn[i]) tarjan(i);
	
	for(int i = 1;i <= n;i++)
		create(i);
	
	bool f = false;	
	for(int i = 1;i <= cnt;i++)
		if(!din[i])//如果入度为0
		{
			ans += (1.0 / n);
			if(!dout[i] && ext[i] == 1) f = true;//如果没有出度，并且大小为1，可以不用尝试
		}
	ans -= f * (1.0 / n);
	
	printf("%.6f",1 - ans);
}