今天考的2014年题，这个题本以为能满分，结果一直卡在20分调不出来，求大佬帮忙查下错谢谢

//d2t2
#include<cstdio>
#include<algorithm>
#include<queue>
#include<cstring>
#define rgi register int
inline int read(){
	int x=0; char c=getchar();
	while(c<'0'||c>'9') c=getchar();
	while(c>='0'&&c<='9') x=x*10+c-'0',c=getchar();
	return x;
}
const int maxn=1e4+10;
const int maxm=2e5+10;
struct Edge{
	int to,next;
}e1[maxm],e2[maxm];
int h1[maxn],h2[maxn],cnt;
inline void addedge(int u,int v){
	e1[++cnt].to=v,e1[cnt].next=h1[u],h1[u]=cnt;
	e2[cnt].to=u,e2[cnt].next=h2[v],h2[v]=cnt;
}
int n,m,s,t;
bool ava[maxn];
void dfs(int x){
	ava[x]=true;
	for(int i=h2[x];i;i=e2[i].next){
		int y=e2[i].to;
		if(ava[y]) continue;
		dfs(y);
	}
}
int dis[maxn];
bool vis[maxn];
int bfs(int s){
	memset(dis,0x3f,sizeof(dis));
	std::queue<int> Q;
	if(ava[s]) Q.push(s);
	dis[s]=0;
	vis[s]=true;
	while(!Q.empty()){
		int x=Q.front();
		Q.pop();
		for(int i=h1[x];i;i=e1[i].next){
			int y=e1[i].to;
			if(vis[y]||!ava[y]) continue;
			dis[y]=dis[x]+1;
			vis[y]=true;
			Q.push(y);
		}
	}
	return dis[t]==0x3f3f3f3f?-1:dis[t];
} 
int main(){
//	freopen("hhh.in","r",stdin);
//	freopen("road.out","w",stdout);
	n=read(),m=read();
	for(rgi i=1;i<=m;i++){
		int x=read(),y=read();
		addedge(x,y);
	}
	s=read(),t=read();
	dfs(t);
	for(rgi i=1;i<=n;i++){
		if(!ava[i]) continue;
		for(int j=h1[i];j;j=e1[j].next)
			if(!ava[e1[j].to]){
				ava[i]=false;
				break;
			}
	}
	printf("%d",bfs(s));
	return 0;
}