题目链接

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>
using namespace std;
#define x first
#define y second
typedef pair<int, int> PII;
const int N = 200010;
int n,m;
int h[N],e[N],ne[N],idx=0;
int st,ed;
int dis[N];
void add(int a,int b)
{
    e[idx]=b,ne[idx]=h[a],h[a]=idx++;
}
int bfs(int st,int ed){
    memset(dis,-1,sizeof dis);
    queue<int>q;
    q.push(st);
    dis[st]=0;
    while(q.size())
    {
        int t=q.front();
        q.pop();
        bool flag=true;
        for(int i=h[t];i!=-1;i=ne[i])
        {
            int j=e[i];
            if(h[j]==-1&&j!=ed)
            {
                flag=false;
                break;
            }
        }
        if(flag)
        {
            for(int i=h[t];i!=-1;i=ne[i])
            {
            int j=e[i];
            if(dis[j]!=-1)continue;
            dis[j]=dis[t]+1;
            if(j==ed)return dis[t]+1;
            q.push(j);
            }
        }
        
    }
    return dis[ed];
}
int main()
{
    cin>>n>>m;
    memset(h,-1,sizeof h);
    for(int i=1;i<=m;i++)
    {
        int a,b;
        scanf("%d%d",&a,&b);
        add(a,b);
    }
    cin>>st>>ed;
    cout<<bfs(st,ed)<<endl;
    
}