1 #include <bits/stdc++.h>
2 #define _for(i,a,b) for(int i = (a);i < b;i ++)
3 #define _rep(i,a,b) for(int i = (a);i > b;i --)
4 #define INF 0x3f3f3f3f
5 #define MOD 1000000007
6 #define maxn 200003
7 typedef long long ll;
8
9 using namespace std;
10 typedef pair<int,int> P;//first 是最短距离，second 是顶点编号
11 inline ll read()
12 {
13 ll ans = 0;
14 char ch = getchar(), last = ' ';
15 while(!isdigit(ch)) last = ch, ch = getchar();
16 while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
17 if(last == '-') ans = -ans;
18 return ans;
19 }
20 inline void write(ll x)
21 {
22 if(x < 0) x = -x, putchar('-');
23 if(x >= 10) write(x / 10);
24 putchar(x % 10 + '0');
25 }
26 int V,E;
27 int st,ed;
28 int ver[maxn],Next[maxn],head[maxn],val[maxn];
29 int ver1[maxn],Next1[maxn],head1[maxn],val1[maxn];
30 int novis[maxn];
31 int cango[maxn];
32 int vis[maxn];
33 int tot = 0,tot1 = 0;
34 void add(int x,int y,int w)
35 {
36 ver[++tot] = y,Next[tot] = head[x],head[x] = tot,val[tot] = w;
37 }
38 void add1(int x,int y,int w)
39 {
40 ver1[++tot1] = y,Next1[tot1] = head1[x],head1[x] = tot1,val1[tot1] = w;
41 }
42 int d[maxn];
43 void shortest_path(int s)
44 {
45 priority_queue<P,vector<P>,greater<P>> que;
46
47 _for(i,1,V+1)
48 d[i] = INF;
49 d[s] = 0;
50 que.push(P {0,s});
51
52 while(!que.empty())
53 {
54 P p = que.top();
55 que.pop();
56 int v = p.second;
57 if(novis[v] || d[v] < p.first) continue;
58 for(int i = head[v]; i; i = Next[i])
59 {
60 int y = ver[i];
61 int w = val[i];
62 if(d[y] > d[v] + w)
63 {
64 d[y] = d[v] + w;
65 que.push(P {d[y],y});
66 }
67 }
68 }
69 }
70
71 bool dfs(int s)
72 {
73 cango[s] = 1;
74 for(int i = head1[s]; i; i = Next1[i])
75 {
76 int y = ver1[i];
77 if(!vis[y])
78 {
79 vis[y] = 1;
80 dfs(y);
81 }
82 }
83 }
84 bool dfs2(int s)
85 {
86 for(int i = head[s]; i; i = Next[i])
87 {
88 int y = ver[i];
89 if(!cango[y])
90 novis[s] = 1;
91 if(!vis[y])
92 {
93 vis[y] = 1;
94 dfs2(y);
95 }
96 }
97 }
98 int main()
99 {
100 // freopen("testdata.in","r+",stdin);
101 V = read(),E = read();
102 _for(i,1,E+1)
103 {
104 int a = read();
105 int b = read();
106 add(a,b,1);
107 add1(b,a,1);
108 }
109 st = read();
110 ed = read();
111 dfs(ed);
112 memset(vis,0,sizeof(vis));
113 dfs2(st);
114 /*
115 printf("\n");
116 _for(i,1,V+1)
117 printf("%d %d %d\n",i,novis[i],cango[i]);
118 */
119
120 shortest_path(st);
121 if(d[ed]==INF)
122 printf("%d\n",-1);
123 else
124 printf("%d\n",d[ed]);
125 return 0;
126 }
