#include<bits/stdc++.h>
using namespace std;
const int maxn=10010,maxm=200010;
int n,m,head[maxn],tail[maxn],s,t,e=0,r1,r2,r3,d[maxn];
bool vis[maxn],bbb[maxn]={};
struct edge
{
	int from,to,next1,next2,val;
	bool bb;
}ed[maxm];

inline void addedge(int x,int y)
{
	ed[++e].to=y;
	ed[e].from=x;
	ed[e].next1=head[x];
	head[x]=e;
	ed[e].next2=tail[y];
	tail[y]=e;
	ed[e].val=1;
}
void dfs1(int x)
{
	vis[x]=1;
	for(int i=tail[x];i;i=ed[i].next2)
	{
		int y=ed[i].from;
		ed[i].bb=1;
		if(vis[y])continue;
		dfs1(y);
	}
}
inline void spfa()
{
	memset(vis,false,sizeof(vis));
	memset(d,0x3f3f3f3f,sizeof(d));
	if(bbb[s])return;
	queue<int> q;
	q.push(s);
	vis[s]=1;
	d[s]=0;
	while(!q.empty())
	{
		int x=q.front();
		q.pop();
		vis[x]=0;
		for(int i=head[x];i;i=ed[i].next1)
		{
			int y=ed[i].to;
			if(bbb[y])continue;
			if(d[y]>d[x]+ed[i].val)
			{
				d[y]=d[x]+ed[i].val;
				if(!vis[y])
				{
					q.push(y);
					vis[y]=1;
				}
			}
		}
	}
}

int main()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++)
	{
		scanf("%d%d",&r1,&r2);
		if(r1==r2)continue;
		addedge(r1,r2);
	}
	scanf("%d%d",&s,&t);
	memset(vis,false,sizeof(vis));
	dfs1(t);
	for(int i=1;i<=n;i++)
	{
		for(int j=head[i];j;j=ed[j].next1)
			if(!ed[j].bb)bbb[i]=1;
	}
	spfa();
	if(d[t]>=0x3f3f3f3f)printf("-1");
	else printf("%d",d[t]);
	return 0;
}