這題挺好的吧~

先處理出不能走的點，然後再跑最短路的時候去判斷就行。

對於不能走的點：

先建反圖跑最短路，給不能走到終點的點打上標記，然後和這些點相連的點也打上標記。

// Author: levil
#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int,int> pii;
const int N = 1e4+5;
const int M = 2e5+5;
const LL Mod = 1e9+7;
#define rg register
#define pi acos(-1)
#define
 
 INF 1e9
#define CT0 cin.tie(0),cout.tie(0)
#define IO ios::sync_with_stdio(false)
#define dbg(ax) cout << "now this num is " << ax << endl;
namespace FASTIO{
    inline LL read(){
        LL x = 0,f = 1;char c = getchar();
        while(c < '0' || c > '9'){if(c == '-') f = -1;c = getchar();}
        
 
while(c >= '0' && c <= '9'){x = (x<<1)+(x<<3)+(c^48);c = getchar();}
        return x*f;
    }
    void print(int x){
        if(x < 0){x = -x;putchar('-');}
        if(x > 9) print(x/10);
        putchar(x%10+'0');
    }
}
using namespace FASTIO;
void FRE(){/*freopen("data1.in","r",stdin);
freopen("data1.out","w",stdout);
 
*/}

int n,m,dis[N],vis[N],s,t;
vector<int> e[N],G[N];
void dij()
{
    for(rg int i = 1;i <= n;++i) dis[i] = INF;
    dis[t] = 0;
    priority_queue<pii,vector<pii>,greater<pii> >Q;
    Q.push(pii{0,t});
    while(!Q.empty())
    {
        int u = Q.top().second;
        int d = Q.top().first;
        Q.pop();
        if(d > dis[u]) continue;
        for(auto v : G[u])
        {
            if(dis[v] > dis[u]+1)
            {
                dis[v] = dis[u]+1;
                Q.push(pii{dis[v],v});
            }
        }
    }
}
void dij2()
{
    for(rg int i = 1;i <= n;++i) dis[i] = INF;
    dis[s] = 0;
    priority_queue<pii,vector<pii>,greater<pii> >Q;
    Q.push(pii{0,s});
    while(!Q.empty())
    {
        int u = Q.top().second;
        int d = Q.top().first;
        Q.pop();
        if(d > dis[u]) continue;
        for(auto v : e[u])
        {
            if(vis[v]) continue;
            if(dis[v] > dis[u]+1)
            {
                dis[v] = dis[u]+1;
                Q.push(pii{dis[v],v});
            }
        }
    }
}
int main()
{
    n = read(),m = read();
    for(rg int i = 1;i <= m;++i)
    {
        int x,y;x = read(),y = read();
        e[x].push_back(y);
        G[y].push_back(x);
    }
    s = read(),t = read();
    dij();
    for(rg int i = 1;i <= n;++i)
    {
        if(dis[i] != INF) continue;
        vis[i] = 1;
        for(auto v : G[i]) vis[v] = 1;
    }
    if(vis[s]) printf("-1\n");
    else 
    {
        dij2();
        if(dis[t] == INF) printf("-1\n");
        else printf("%d\n",dis[t]);
    }
    system("pause");    
}