#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
//using namespace atcoder;
using ll = long long;
using ull = unsigned long long;
using i128 = __int128_t;
using u128 = unsigned __int128_t;
using mint = atcoder::static_modint<998244353>;
const int mod = 998244353;
#include <chrono>
mt19937_64 rng(std::chrono::steady_clock::now().time_since_epoch().count());
int dx[8] = {-1, 1, 0, 0, -1, -1, 1, 1};
int dy[8] = {0, 0, -1, 1, -1, 1, -1, 1};
template<class T>
bool chmin(T& a, const T& b){
    if (b < a){
        a = b;
        return true;
    }
    else { 
        return false;
    }
}
template<class T> 
bool chmax(T& a, const T& b){
    if (a < b){
        a = b;
        return true;
    }
    else {
        return false;
    }
}

void solve(){
    int n, m, a, b;
    cin >> n >> m >> a >> b;
    a--;
    vector<vector<int>> vec(n + 1);
    for (int i = 0; i < m; i++){
        int u, v;
        cin >> u >> v;
        u--;
        vec[u].push_back(v);
        vec[v].push_back(u);
    }
    vector<int> dist(n + 1, 1e9);
    queue<int> q;
    for (int i = 0; i <= a; i++){
        q.push(i);
        dist[i] = 0;
    }
    while(!q.empty()){
        int x = q.front();
        q.pop();
        for (auto d : vec[x]){
            if (dist[d] > dist[x] + 1){
                dist[d] = dist[x] + 1;
                q.push(d);
            }
        }
    }
    int ans = 1e9;
    for (int i = b; i <= n; i++){
        chmin(ans, dist[i]);
    }
    cout << (ans == 1e9 ? -1 : ans) << '\n';
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int t = 1;   
    while(t--){
        cout << fixed << setprecision(15);          
        solve();
    }   
}