#include <bits/stdc++.h>
using namespace std;
using ll=long long;
const ll ILL=2167167167167167167;
const int INF=2100000000;
#define rep(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define all(p) p.begin(),p.end()
template<class T> using pq_ = priority_queue<T, vector<T>, greater<T>>;
template<class T> int LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> int UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,T b){if(b<a){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,T b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
bool yneos(bool a,bool upp=false){if(a){cout<<(upp?"YES\n":"Yes\n");}else{cout<<(upp?"NO\n":"No\n");}return a;}
template<class T> void vec_out(vector<T> &p,int ty=0){
    if(ty==2){cout<<'{';for(int i=0;i<(int)p.size();i++){if(i){cout<<",";}cout<<'"'<<p[i]<<'"';}cout<<"}\n";}
    else{if(ty==1){cout<<p.size()<<"\n";}for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}}
template<class T> T vec_min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;}
template<class T> T vec_max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;}
template<class T> T vec_sum(vector<T> &a){T ans=T(0);for(auto &x:a) ans+=x;return ans;}
int pop_count(long long a){int res=0;while(a){res+=(int)(a&1),a>>=1;}return res;}
template<class T> T square(T a){return a * a;}

namespace po167{
    struct UFtree_undo{
        int _n,ind;
        std::vector<int> wei;
        std::vector<int> q;
        int component;
        UFtree_undo(int n):_n(n),ind(0),wei(n),component(n),par(n){
            for(int i=0;i<n;i++){
                wei[i]=1,par[i]=i;
            }
        }
        int undo(){
            if(ind==0) return 0;
            component++;
            ind--;
            wei[par[q[ind]]]-=wei[q[ind]];
            par[q[ind]]=q[ind];
            return 1;
        }
        void rollback(int times){
            int tmp=1;
            while(tmp!=0&&times!=0){
                tmp=undo();
                times--;
            }
        }
        void intialize(){
            while(ind) undo();
        }
        //根っこを返す
        int root(int a){
            assert(0<=a&&a<_n);
            if(a==par[a]) return a;
            return root(par[a]);
        }
        //trueなら1,falseなら0
        int same(int a,int b){
            assert(0<=a&&a<_n);
            assert(0<=b&&b<_n);
            if(root(a)==root(b)) return 1;
            else return 0;
        }
        //a,bが違う根っこの元なら結合する,結合したらtrueを返す
        bool unite(int a,int b){
            a=root(a),b=root(b);
            if(a==b) return false;
            if(wei[a]<wei[b]) std::swap(a,b);
            par[b]=a;
            if(ind==(int)(q.size())) q.push_back(b);
            else q[ind]=b;
            ind++;
            wei[a]+=wei[b];
            component--;
            return true;
        }
        int size(int a){
            return wei[root(a)];
        }
    private:
        std::vector<int> par;
    };
}
using po167::UFtree_undo;

using len_type=pair<int, int>;
struct edge{
    int to;
    len_type length;
};

len_type dijkstra_movement(edge e,int from,len_type t){
    return {t.first + e.length.first, t.second + e.length.second};
}


vector<len_type> dijkstra(vector<vector<edge>> &G,int start,len_type e, len_type inf){
    int M=G.size();
    assert(M>start&&start>=0);
    vector<len_type> seen(M,inf);
    seen[start]=e;
    priority_queue<pair<len_type,int>,vector<pair<len_type,int>>,greater<pair<len_type,int>>> pq;
    pq.push({e,start});
    while(!pq.empty()){
        len_type time;
        int ver;
        tie(time,ver)=pq.top();
        pq.pop();
        if(seen[ver]<time) continue;
        for(edge x:G[ver]){
            len_type T=dijkstra_movement(x,ver,time);
            if(T<seen[x.to]){
                pq.push({T,x.to});
                seen[x.to]=T;
            }
        }
    }
    return seen;
}


void solve();
// DEAR MYSTERIES / TOMOO
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    rep(i, 0, t) solve();
}

void solve(){
    int V, E, st, en;
    cin >> V >> E >> st >> en;
    st--, en--;
    vector<int> A(E), B(E);
    rep(i, 0, E) cin >> A[i] >> B[i], A[i]--, B[i]--;
    vector<int> cy(E);
    {
        UFtree_undo T(V);
        auto f = [&](auto self, int l, int r) -> void {
            if (l + 1 == r){
                if (T.same(A[l], B[l])) cy[l] = 1;
                return;
            }
            int m = (l + r) / 2;
            pair<int, int> X = {l, m}, Y = {m, r};
            rep(rp, 0, 2){
                swap(X, Y);
                int C = 0;
                rep(i, X.first, X.second) if (T.unite(A[i], B[i])) C++;
                self(self, Y.first, Y.second);
                T.rollback(C);
            }
        };
        f(f, 0, E);
    }
    vector<vector<edge>> G(V);
    rep(i, 0, E){
        G[A[i]].push_back({B[i], {1, cy[i]}});
        G[B[i]].push_back({A[i], {1, cy[i]}});
    }
    auto seen = dijkstra(G, st, {0, 0}, {INF, -1});
    cout << seen[en].second << "\n";
}