#define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define MT make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define RT return
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;

vector<int> bfs(const vector<vector<int>> &G, vi &S) {
    vector<int> res(G.size(), -1);
    queue<pair<int, int> > Q;
    each(start, S)Q.push(make_pair(start, 0));
    while (Q.size()) {
        auto p = Q.front();
        Q.pop();
        if (res[p.first] != -1) continue;
        res[p.first] = p.second;
        for (auto v : G[p.first])
            Q.push(make_pair(v, p.second + 1));
    }
    return res;
}

void solve() {
    int N, M, A, B;
    cin >> N >> M >> A >> B;
    vector<vi> G(N + 1);
    rep(i, M) {
        int L, R;
        cin >> L >> R;
        --L;
        G[L].push_back(R);
        G[R].push_back(L);
    }

    vi S;
    rep(i, A)S.push_back(i);
    vi D = bfs(G, S);

    int ans = INT_MAX;
    for (int i = B; i < N + 1; ++i) {
        if (D[i] != -1) {
            smin(ans, D[i]);
        }
    }
    if (ans == INT_MAX)ans = -1;
    cout << ans << endl;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(15);
    solve();
    return 0;
}