#include<bits/stdc++.h>
using namespace std;
using ll=long long;

#include<atcoder/math>
using namespace atcoder;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N,M,S,T;
    cin>>N>>M>>S>>T;
    S--;T--;
    vector<vector<int>> G(N);
    vector<int> C(N,0);
    set<array<int,2>> EF;
    for(int i=0;i<M;i++){
        int u,v;
        cin>>u>>v;
        u--;v--;
        if(u>v)swap(u,v);
        G[u].push_back(v);
        G[v].push_back(u);
        EF.insert({u,v});
        C[u]++;
        C[v]++;
    } 
    queue<int> Q;
    for(int i=0;i<N;i++){
        if(C[i]==1)Q.push(i);
    }
    while(!Q.empty()){
        int p=Q.front();
        Q.pop();
        for(int v:G[p]){
            EF.erase({min(p,v),max(p,v)});
            C[v]--;
            if(C[v]==1)Q.push(v);
        }
    }
    const int INF=1e9;
    vector<array<int,2>> D(N,{INF,INF});
    D[S]={0,0};
    priority_queue<array<int,3>,vector<array<int,3>>,greater<array<int,3>>> que;
    que.push({0,0,S});
    
    vector<bool> seen(N,0);
    while(!que.empty()){
        auto [c,d,n]=que.top();
        que.pop();
        if(seen[n])continue;
        seen[n]=1;
        for(auto v:G[n]){
            int nc=c+1;
            int nd=d-(EF.count({min(n,v),max(n,v)}));
            if(seen[v])continue;
            array<int, 2> ND={nc,nd};
            // cout<<c<<" "<<d<<" "<<n<<" "<<nc<<" "<<nd<<" "<<v<<endl;
            if(D[v]<=ND)continue;
            D[v]=ND;
            que.push({nc,nd,v});
        }
    }
    if(D[T][0]>1e7)cout<<-1<<"\n";
    else cout<<-D[T][1]<<"\n";
}