#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<int> D(N,INF);
    vector<int> R(N,-1);
    D[S]=0;
    queue<int> que; 
    que.push(0);
    
    vector<bool> seen(N,0);
    while(!que.empty()){
        int n=que.front();
        que.pop();
        if(seen[n])continue;
        seen[n]=1;
        for(auto v:G[n]){
            if(seen[v])continue;
            if(D[v]<=D[n]+1)continue;
            D[v]=D[n]+1;
            R[v]=n;
            que.push(v);
        }
    }
    if(D[T]>1e7)cout<<-1<<"\n";
    else{
        int an=0;
        while(T!=S){
            int Y=R[T];
            if(EF.count({min(Y,T),max(Y,T)}))an++;
            T=Y;
        }
        cout<<an<<"\n";
    }
}