#ifdef ONLINE_JUDGE
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#endif
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rrep(i,start,end) for (ll i = start;i >= (ll)(end);i--)
#define repn(i,end) for(ll i = 0; i <= (ll)(end); i++)
#define reps(i,start,end) for(ll i = start; i < (ll)(end); i++)
#define repsn(i,start,end) for(ll i = start; i <= (ll)(end); i++)
#define each(p,a) for(auto &p:a)
typedef vector<ll> vll;
typedef vector<pair<ll ,ll>> vpll;
typedef vector<vector<pair<ll ,ll>>> vvpll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;
typedef set<ll> sll;
typedef map<ll , ll> mpll;
typedef pair<ll ,ll> pll;
typedef tuple<ll , ll , ll> tpl3;
typedef tuple<ll , ll , ll , ll> tpl4;
typedef tuple<ll , ll , ll , ll , ll> tpl5;
typedef tuple<ll , ll , ll , ll , ll , ll> tpl6;
#define LL(...) ll __VA_ARGS__; input(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__; input(__VA_ARGS__)
#define Str(...) string __VA_ARGS__; input(__VA_ARGS__)
#define Ch(...) char __VA_ARGS__; input(__VA_ARGS__)
#define all(a)  (a).begin(),(a).end()
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
#define sz(x) (ll)x.size()
// << std::fixed << std::setprecision(10)
const ll INF = 1LL << 60;
const ld EPS = 1e-9;
 
ll lceil(ll a,ll b){if(a%b==0){return a/b;}if(a>=0){return (a/b)+1;}else{return -((-a)/b);}}
ll lfloor(ll a,ll b){if(a%b==0){return a/b;}if(a>=0){return (a/b);}else{return -((-a)/b)-1;}}
inline ll positive_mod(ll a,ll m){return (a % m + m)%m;}
inline ll popcnt(ull a){ return __builtin_popcountll(a);}
//0indexed
inline ll topbit(ll a){assert(a != 0);return 63 - __builtin_clzll(a);}
inline ll smlbit(ll a){assert(a != 0);return __builtin_ctzll(a);}
template<class T> bool chmin(T& a, T b){if(a > b){a = b;return true;}return false;}
template<class T> bool chmax(T& a, T b){if(a < b){a = b;return true;}return false;}
template<typename T> std::istream &operator>>(std::istream&is,std::vector<T>&v){for(T &in:v){is>>in;}return is;}
template<typename T> std::ostream &operator<<(std::ostream&os,const std::vector<T>&v){for(auto it=std::begin(v);it!=std::end(v);){os<<*it<<((++it)!=std::end(v)?" ":"");}return os;}
template<typename T1, typename T2>std::ostream &operator<< (std::ostream &os, std::pair<T1,T2> p){os << "{" << p.first << "," << p.second << "}";return os;}
template<class... T>void input(T&... a){(cin >> ... >> a);}
void print(){cout << endl;}
template<class T, class... Ts>void print(const T& a, const Ts&... b){cout << a;((cout << ' ' << b), ...);cout << endl;}
template<class T> void pspace(const T& a){ cout << a << ' ';}
void perr(){cerr << endl;}
template<class T, class... Ts>void perr(const T& a, const Ts&... b){cerr << a;((cerr << ' ' << b), ...);cerr << endl;}
void yes(bool i = true){ return print(i?"yes":"no"); }
void Yes(bool i = true){ return print(i?"Yes":"No"); }
void YES(bool i = true){ return print(i?"YES":"NO"); }
template <class T> vector<T> &operator++(vector<T> &v) {for(auto &e : v) e++;return v;}
template <class T> vector<T> operator++(vector<T> &v, signed) {auto res = v;for(auto &e : v) e++;return res;}
template <class T> vector<T> &operator--(vector<T> &v) {for(auto &e : v) e--;return v;}
template <class T> vector<T> operator--(vector<T> &v, signed) {auto res = v;for(auto &e : v) e--;return res;}
//grid探索用
vector<ll> _ta = {0,0,1,-1,1,1,-1,-1};
vector<ll> _yo = {1,-1,0,0,1,-1,1,-1};
bool isin(ll now_i,ll now_j,ll h,ll w){return (0<=now_i && now_i < h && 0 <= now_j && now_j < w);}
  
ll lpow(ll x,ll n){ll ans = 1;while(n >0){if(n & 1)ans *= x;x *= x;n >>= 1;}return ans;}
ll Modlpow(ll x,ll n,ll m){ll ans = 1;ll a = x%m;while(n >0){if(n & 1){ans *= a;ans%= m;}a *= a;a %= m;n >>= 1;}return ans;} 
const ll MOD9 = 998244353LL;
const ll MOD10 = 1000000007LL;

struct Lowlink{
  ll n;
  vector<ll> ord;
  vector<ll>low;
  vector<ll>articulation,bridge;//atriculation 関節点,bridgeは橋になる辺の番号を格納 
  vector<vector<pll>> g;
  vector<ll>found;
  vector<ll> c;//二重辺連結分解で使用
  ll m;//辺の数
 
  Lowlink(ll _n):n(_n),m(0){
    ord.resize(n,INF);
    low.resize(n);
    g.resize(n);
    found.resize(n);
  }
 
  ll add_edge(ll u, ll v){
    g[u].push_back({v,m});
    g[v].push_back({u,m});
    return m++;
  }
 
  ll dfs(ll v,ll par, ll num){
    ord[v] = num;
    low[v] = num;
    found[v] =1;
    bool isa = false; //関節点かどうか
    ll cnt = 0;//ｖが根のときに関節点かどうか判定に使用
    for(auto &p:g[v]){
      auto[to,id]  = p;
      if(to == par){continue;}
      if(found[to] != 0){
        //発見済みだったから今見ている辺は後退辺
        chmin(low[v],ord[to]);
      }else{
        //未発見
        cnt++;
        num = dfs(to,v,num+1);
        if(par != -1 && low[to] >= ord[v]){
          isa = true;
        }
        chmin(low[v],low[to]);
        if(ord[v] < low[to]){
          //子のlowが子で終わってたらbridge
          bridge.push_back(id);
        }
      }
    }
    if(par == -1  && cnt >= 2)isa = true;
    if(isa)articulation.push_back(v);//関節点だった
    return num + 1;
  }
  //構築して連結成分数を返す 
  ll build(){
    ll k = 0;
    ll cnt = 0;//シンプルに連結成分数
    rep(i,n){
      if(found[i] == 0){
        k = dfs(i,-1,k);
        cnt++;
      }
    }
    return cnt; //シンプルに連結成分数を返しておく
  }
 
  vector<vector<ll>> bc;
  vector<ll> bc_id;
  //二重頂点連結分解
  //関節点で切る。辺の集合が得られる。
  void bcc(){
    bc_id = vector<ll>(m,-1);
    vector<ll> used(n);
    auto add = [&](ll id,ll num){
      bc[num].push_back(id);
      bc_id[id] = num;
    };
    auto dfs = [&](auto dfs,ll v,ll par,ll num)->void{
      used[v] = 1;
      for(auto &[to,id]:g[v]){
        if(to == par){continue;}
        if(used[to] == 0 ){
          ll nownum = num;
          if(low[to] >= ord[v]){
            nownum = bc.size();
            bc.push_back({});//新しくグループ作る
          }
          add(id,nownum);
          dfs(dfs,to,v,nownum);
        }else if(ord[to] < ord[v]){
          add(id,num);//後退辺は同じグループ
        }
      }
    };
    rep(i,n){
      if(used[i] == 0){
        dfs(dfs,i,-1,-1);
      }
    }
  }

  

  //二重辺連結分解 build()した後
  //cに各頂点がどのグループに属するか格納される
  //橋を含まない頂点集合に分解する
  ll two_edge_connect_component(){
    //~~~~~~二重辺連結分解~~~~~~~~~~~
    // 計算量 O(n+m)
    c.resize(n,-1);
    ll ccnt = 0;
    rep(i,n){
      if(c[i] == -1){
        auto dfs2 = [&](auto dfs2, ll v,ll par,ll num)->ll{
          if(par != -1 && ord[par] >= low[v]){
            c[v] = c[par];
          }else{
            c[v] = num++;
          }
          for(auto &[to,id]:g[v]){
            if(c[to] == -1){
              num = dfs2(dfs2,to,v,num);
            }
          }
          return num;
        };
        ccnt = dfs2(dfs2,i,-1,ccnt);
      }
    }
    return ccnt;
  }
};
 
int main(){
  ios::sync_with_stdio(false);cin.tie(nullptr);
  LL(n,m,s,t);
  s--;t--;
  vvll g(n);
  Lowlink lw(n);
  map<pll,ll> mp;
  rep(i,m){
    LL(a,b);
    a--;b--;
    g[a].push_back(b);
    g[b].push_back(a);
    lw.add_edge(a,b);
    mp[{a,b}] = i;
    mp[{b,a}] = i;
  }

  lw.build();
  set<ll> bridges;
  each(p,lw.bridge){
    bridges.insert(p);
  }
  queue<ll> que;
  //0からの距離が入る
  //01BFSのときは更新の判定のところを変更する
  vector<ll> find(n,INF);
  vll ans(n,0);
  find[s] = 0;
  que.push(s);
  while(!que.empty()){
    ll now = que.front();
    que.pop();
    for(auto &p:g[now]){
      if(find[p] > find[now]+1){
        find[p] = find[now]+1;
        que.push(p);
      }else if(find[p] == find[now]-1){
        ll id = mp[{now,p}];
        if(bridges.contains(id)){
          chmax(ans[now],ans[p]);
        }else{
          chmax(ans[now],ans[p]+1);
        }
      }
    }
  }
  if(find[t] == INF){
    cout << -1 << endl;
  }else{
    cout << ans[t] << endl;
  }

}