#include <bits/stdc++.h>

 using namespace std;

 #define INIT() ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);

 #define Rep(i,n) for (int i = 0; i < (n); ++i)

 #define For(i,s,t) for (int i = (s); i <= (t); ++i)

 #define rFor(i,t,s) for (int i = (t); i >= (s); --i)

 #define ForLL(i, s, t) for (LL i = LL(s); i <= LL(t); ++i)

 #define rForLL(i, t, s) for (LL i = LL(t); i >= LL(s); --i)

 #define foreach(i,c) for (__typeof(c.begin()) i = c.begin(); i != c.end(); ++i)

 #define rforeach(i,c) for (__typeof(c.rbegin()) i = c.rbegin(); i != c.rend(); ++i)

 #define pr(x) cout << #x << " = " << x << "  "

 #define prln(x) cout << #x << " = " << x << endl

 #define LOWBIT(x) ((x)&(-x))

 #define ALL(x) x.begin(),x.end()

 #define INS(x) inserter(x,x.begin())

 #define UNIQUE(x) x.erase(unique(x.begin(), x.end()), x.end())

 #define REMOVE(x, c) x.erase(remove(x.begin(), x.end(), c), x.end()); // 删去 x 中所有 c

 #define TOLOWER(x) transform(x.begin(), x.end(), x.begin(),::tolower);

 #define TOUPPER(x) transform(x.begin(), x.end(), x.begin(),::toupper);

 #define ms0(a) memset(a,0,sizeof(a))

 #define msI(a) memset(a,inf,sizeof(a))

 #define msM(a) memset(a,-1,sizeof(a))

 #define MP make_pair

 #define PB push_back

 #define ft first

 #define sd second

 template<typename T1, typename T2>

 istream &operator>>(istream &in, pair<T1, T2> &p) {

     in >> p.first >> p.second;

     return in;

 }

 template<typename T>

 istream &operator>>(istream &in, vector<T> &v) {

     for (auto &x: v)

         in >> x;

     return in;

 }

 template<typename T1, typename T2>

 ostream &operator<<(ostream &out, const std::pair<T1, T2> &p) {

     out << "[" << p.first << ", " << p.second << "]" << "\n";

     return out;

 }

 inline int gc(){

     static const int BUF = 1e7;

     static char buf[BUF], *bg = buf + BUF, *ed = bg;

     if(bg == ed) fread(bg = buf, , BUF, stdin);

     return *bg++;

 } 

 inline int ri(){

     int x = , f = , c = gc();

     for(; c<||c>; f = c=='-'?-:f, c=gc());

     for(; c>&&c<; x = x* + c - , c=gc());

     return x*f;

 }

 template<class T>

 inline string toString(T x) {

     ostringstream sout;

     sout << x;

     return sout.str();

 }

 inline int toInt(string s) {

     int v;

     istringstream sin(s);

     sin >> v;

     return v;

 }

 //min <= aim <= max

 template<typename T>

 inline bool BETWEEN(const T aim, const T min, const T max) {

     return min <= aim && aim <= max;

 }

 typedef long long LL;

 typedef unsigned long long uLL;

 typedef pair< double, double > PDD;

 typedef pair< int, int > PII;

 typedef pair< int, PII > PIPII;

 typedef pair< string, int > PSI;

 typedef pair< int, PSI > PIPSI;

 typedef set< int > SI;

 typedef set< PII > SPII;

 typedef vector< int > VI;

 typedef vector< double > VD;

 typedef vector< VI > VVI;

 typedef vector< SI > VSI;

 typedef vector< PII > VPII;

 typedef map< int, int > MII;

 typedef map< int, string > MIS;

 typedef map< int, PII > MIPII;

 typedef map< PII, int > MPIII;

 typedef map< string, int > MSI;

 typedef map< string, string > MSS;

 typedef map< PII, string > MPIIS;

 typedef map< PII, PII > MPIIPII;

 typedef multimap< int, int > MMII;

 typedef multimap< string, int > MMSI;

 //typedef unordered_map< int, int > uMII;

 typedef pair< LL, LL > PLL;

 typedef vector< LL > VL;

 typedef vector< VL > VVL;

 typedef priority_queue< int > PQIMax;

 typedef priority_queue< int, VI, greater< int > > PQIMin;

 const double EPS = 1e-;

 const LL inf = 0x7fffffff;

 const LL infLL = 0x7fffffffffffffffLL;

 const LL mod = 1e9 + ;

 const int maxN = 1e5 + ;

 const LL ONE = ;

 const LL evenBits = 0xaaaaaaaaaaaaaaaa;

 const LL oddBits = 0x5555555555555555;

 int N, M, S, T, ans = -;

 VI v[maxN];

 int vis[maxN][];

 void bfs() {

     int cnt = , times = ;

     queue< int > Q;

     Q.push(S);

     vis[S][] = ;    

     while(!Q.empty()) {

         int tmp = Q.front(); Q.pop();

         --cnt;

         foreach(i, v[tmp]) {

             if(vis[*i][]) continue; // 如果从 *i 向下走两步已经执行过，就跳过

             vis[*i][] = ;

             foreach(j, v[*i]) {

                 if(vis[*j][]) continue; // 如果从 *j 向下走一步已经执行过，就跳过

                 vis[*j][] = ;

                 foreach(k, v[*j]) {

                     if(vis[*k][]) continue; // 如果从 *k 向下走零步已经执行过，就跳过

                     vis[*k][] = ;

                     if(*k == T) {

                         ans = times + ;

                         return;

                     }

                     Q.push(*k);

                 }

             }

         }

         if(cnt == ) {

             cnt = Q.size();

             ++times;

         }

     }

 } 

 int main(){

     //freopen("MyOutput.txt","w",stdout);

     //freopen("input.txt","r",stdin);

     INIT();

     cin >> N >> M;

     Rep(i, M) {

         int x, y;

         cin >> x >> y;

         v[x].PB(y);

     }

     cin >> S >> T;

     bfs();

     cout << ans << endl;

     return ;

 }