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(" "); 116 _for(i,1,V+1) 117 printf("%d %d %d ",i,novis[i],cango[i]); 118 */ 119 120 shortest_path(st); 121 if(d[ed]==INF) 122 printf("%d ",-1); 123 else 124 printf("%d ",d[ed]); 125 return 0; 126 }