#include<iostream>

#include<cstdio>

#include<cstring>

#include<queue>

#include<algorithm>

#define il inline

#define Rg register

#define go(i,a,b) for(Rg int i=a;i<=b;++i)

#define yes(i,a,b) for(Rg int i=a;i>=b;--i)

#define e(i,u) for(Rg int i=b[u];i;i=a[i].nt)

#define mem(a,b) memset(a,b,sizeof(a))

#define ll long long

#define db double

#define inf 2147483647

using namespace std;

il int read()

{

    Rg int x=,y=;char c=getchar();

    while(c<''||c>''){if(c=='-')y=-;c=getchar();}

    while(c>=''&&c<=''){x=(x<<)+(x<<)+c-'';c=getchar();}

    return x*y;

}

const int N=,M=;

int n,m,b[N],ct,du[N],s,t,vis[N],dis[N];

bool fl[N];

queue<int>q;

struct nd1{int u,v;}eg[M];

struct nd{int v,nt;}a[M];

il void add(int u,int v){a[++ct]=(nd){v,b[u]};b[u]=ct;}

il void dfs(int u)

{

    e(i,u)

    {

        Rg int v=a[i].v;vis[v]++;

        if(vis[v]>)continue;

        dfs(v);

    }

}

il void SPFA()

{

    mem(vis,);mem(dis,);

    vis[s]=;dis[s]=;q.push(s);

    while(!q.empty())

    {

        Rg int u=q.front();q.pop();vis[u]=;

        e(i,u)

        {

            Rg int v=a[i].v;

            if(!fl[v])continue;

            if(dis[v]>dis[u]+)

            {

                dis[v]=dis[u]+;

                if(!vis[v]){vis[v]=;q.push(v);}

            }

        }

    }

}

il bool cmp(nd1 x,nd1 y){return x.u==y.u?x.v<y.v:x.u<y.u;}

int main()

{

    n=read(),m=read();

    go(i,,m)

    {

        Rg int u=read(),v=read();

        if(u==v)continue;

        eg[i]=(nd1){u,v};//add(v,u);du[u]++;

    }

    sort(eg+,eg+m+,cmp);

    Rg int nm=;

    go(i,,m)

    {

        if(eg[i].u==eg[i-].u && eg[i].v==eg[i-].v){nm++;continue;}

        add(eg[i].v,eg[i].u);du[eg[i].u]++;

    }

    m-=nm;

    s=read(),t=read();swap(s,t);

    vis[s]=;dfs(s);

    go(i,,n)if((vis[i]==du[i]&&vis[i])||i==s)fl[i]=;

    if(!fl[s]){printf("-1\n");return ;}

    SPFA();

    if(dis[t]>M)printf("-1\n");

    else printf("%d\n",dis[t]);

    return ;

}