#include <bits/stdc++.h>

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define FOR(i,n,m) for(int i=(int)(n); i<=(int)(m); i++)
#define RFOR(i,n,m) for(int i=(int)(n); i>=(int)(m); i--)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define RITR(x,c) for(__typeof(c.rbegin()) x=c.rbegin();x!=c.rend();x++)
#define setp(n) fixed << setprecision(n)

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

#define ll long long
#define vll vector<ll>
#define vi vector<int>
#define pll pair<ll,ll>
#define pi pair<int,int>

#define all(a) (a.begin()),(a.end())
#define rall(a) (a.rbegin()),(a.rend())
#define fi first
#define se second
#define pb push_back
#define ins insert

#define debug(a) cerr<<(a)<<endl
#define dbrep(a,n) rep(_i,n) cerr<<(a[_i])<<" "; cerr<<endl
#define dbrep2(a,n,m) rep(_i,n){rep(_j,m) cerr<<(a[_i][_j])<<" "; cerr<<endl;}

using namespace std;

template<class A, class B>
ostream &operator<<(ostream &os, const pair<A,B> &p){return os<<"("<<p.fi<<","<<p.se<<")";}
template<class A, class B>
istream &operator>>(istream &is, pair<A,B> &p){return is>>p.fi>>p.se;}

template<class T>
vector<T> make_vec(size_t a){
	return vector<T>(a);
}
template<class T, class... Ts>
auto make_vec(size_t a, Ts... ts){
	return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

constexpr int INF = 1e9;

//-------------------------------------------------

int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N,M,A,B; cin>>N>>M>>A>>B; A--;
    
    vector<vi> G(N+1);
    rep(i,M){
        int l,r; cin>>l>>r; l--;
        G[l].pb(r);
        G[r].pb(l);
    }

    vi dist(N+1,INF);
    queue<int> que;
    rep(i,N+1){
        if (i <= A){
            que.emplace(i);
            dist[i]=0;
        }
    }

    while(!que.empty()){
        int v = que.front(); que.pop();
        for(auto u:G[v]){
            if (dist[u]!=INF) continue;
            dist[u] = dist[v]+1;
            que.emplace(u);
        }
    }
    
    int ans=INF;
    rep(i,N+1){
        if (i >= B){
            chmin(ans,dist[i]);
        }
    }
    if (ans==INF) cout<<"-1\n";
    else cout<<ans<<"\n";

    return 0;
}