C++ :

#include <cstdio>
#include <cstring>

#define  maxn  10005LL
#define  maxe  200005LL
#define  infi  0x3f3f3f3fLL
#define  sizeque  20000LL

struct FST{
	int to , next ;
} e[maxe] , re[maxe] ;

int tot = 0 , star[maxn] = {0} ;
void AddEdge(int u , int v) {
	tot ++ ; e[tot].to = v ;
	e[tot].next = star[u] ; star[u] = tot ;
}

int rtot = 0 , rstar[maxn] = {0} ;
void AddrEdge(int u , int v) {
	rtot ++ ; re[rtot].to = v ;
	re[rtot].next = rstar[u] ; rstar[u] = rtot ;
}

int n , m ;
int St , En ;
int dis[maxn] = {0} ;
bool exist[maxn] = {0} ;
bool flag[maxn] = {0} ;

struct Que{
	int q[maxn] , head , tail ;
	int F(int x) { return ((x%sizeque)+sizeque)%sizeque ; }
	int front() { return q[F(head)] ; }
	bool empty() { return head > tail ; }
	void clear() { head = 0 ; tail = -1 ; }
	void pop() { head ++ ; }
	void push_front(int x) { q[F(--head)] = x ; }
	void push_back(int x) { q[F(++tail)] = x ; }
} q ;

void rspfa() {
	memset(dis , 0x3f , sizeof (dis)) ;
	dis[En] = 0 ; q.clear() ; q.push_back(En) ;
	int u , v , p ;
	while (!q.empty()) {
		u = q.front() ; q.pop() ; exist[u] = false ;
		for (p=rstar[u],v=re[p].to;p;p=re[p].next,v=re[p].to) {
			if (dis[v] > dis[u]+1) {
				dis[v] = dis[u]+1 ;
				if (!exist[v]) {
					exist[v] = true ;
					if (dis[v] < dis[q.front()]) q.push_front(v) ;
					else q.push_back(v) ;
				}
			}
		}
	}
	for (u = 1 ; u <= n ; u ++ )
		if (dis[u] == infi) {
			flag[u] = true ;
			for (p=rstar[u],v=re[p].to;p;p=re[p].next,v=re[p].to)
				flag[v] = true ;
		}
}

void spfa() {
	if (flag[St]) {
		puts("-1") ; return ;
	}
	memset(dis , 0x3f , sizeof (dis)) ;
	dis[St] = 0 ; q.clear() ; q.push_back(St) ;
	int u , v , p ;
	while (!q.empty()) {
		u = q.front() ; q.pop() ; exist[u] = false ;
		for (p=star[u],v=e[p].to;p;p=e[p].next,v=e[p].to) {
			if (flag[v]) continue ;
			if (dis[v] > dis[u]+1) {
				dis[v] = dis[u]+1 ;
				if (!exist[v]) {
					exist[v] = true ;
					if (dis[v] < dis[q.front()]) q.push_front(v) ;
					else q.push_back(v) ;
				}
			}
		}
	}
	if (dis[En] == infi) puts("-1") ;
	else printf("%d\n" , dis[En] ) ;
}

int main() {
//	    freopen("road.in" ,"r",stdin ) ;
//	    freopen("road.out","w",stdout) ;
	int i , u , v ;
	scanf("%d%d", &n , &m ) ;
	for (i = 0 ; i < m ; i ++ ) {
		scanf("%d%d", &u , &v ) ;
		AddEdge(u , v) ;
		AddrEdge(v , u) ;
	}
	scanf("%d%d", &St , &En ) ;
	rspfa() ;
	spfa() ;
	    fclose(stdin ) ; fclose(stdout) ;
	return 0 ;
}