#include<bits/stdc++.h>
using namespace std;
#define int long long
int n,m,s,t,vis[200005],hv[200005],dis[200005];
vector<int>g[200005],FG[200005];
void _BFS(){
	queue<int>q;
	q.push(t);
	vis[t]=1;
	while(!q.empty()){
		int x=q.front();
		q.pop();
		for(int v:FG[x]){
			if(!vis[v]){
				q.push(v);
				vis[v]=1;
			}
		}
	}
}
void _FIND(){
	for(int i=1;i<=n;i++){
		if(!vis[i])
			continue;
		for(auto v:g[i]){
			if(!vis[v])
				break;
			hv[i]=1;
		}
	}
}
bool BFS_(){
	queue<int>q;
	q.push(s);
	while(!q.empty()){
		int x=q.front();
		q.pop();
		for(auto v:g[x]){
			if(hv[v]){
				q.push(v);
				dis[v]=dis[x]+1;
				if(v==t){
					cout<<dis[t];
					return 1;
				}
			}
		}
	}
	return 0;
}
signed main(){
	ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		int u,v;
		cin>>u>>v;
		g[u].push_back(v);
		FG[v].push_back(u);
	}
	cin>>s>>t;
	_BFS();
	_FIND();
	if(!BFS_())
		cout<<-1;
	return 0;
}