// (◕ᴗ◕✿)

// #pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define srep(i, s, n) for (ll i = s; i < (n); i++)
#define len(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
using namespace std;
template<typename T> using vc = vector<T>;
template<typename T> using vv = vc<vc<T>>;
template<typename T> using vvv = vv<vc<T>>;
using vi = vc<int>;using vvi = vv<int>; using vvvi = vv<vi>;
using ll = long long;using vl = vc<ll>;using vvl = vv<ll>; using vvvl = vv<vl>;
using ld = long double; using vld = vc<ld>; using vvld = vc<vld>; using vvvld = vc<vvld>;
using uint = unsigned int;
using ull = unsigned long long;
const ld pi = acos(-1.0);
const int inf = 0x3f3f3f3f;
const ll INF = 0x3f3f3f3f3f3f3f3f;
// const ll mod = 1000000007;
const ll mod = 998244353;
inline bool inside(ll y, ll x, ll H, ll W) {return 0 <= (y) and (y) < (H) and 0 <= (x) and (x) < (W); }

#define debug(var) do { cerr << #var << " :\n"; view(var); } while(0)
template<typename T>void view(const T& e) {cerr << e;}
template<typename T1, typename T2>void view(const pair<T1, T2>& p) {cerr << "{" << p.first << ", " << p.second << "}";}
template<typename T>void view(const vc<T>& v) {for (const auto& e : v) {view(e);cerr << " ";} cerr << endl;}
template<typename T>void view(const vv<T>& vv) {for (const auto& v : vv) {view(v);} cerr << endl;}
template<typename T>void view(const set<T>& s) {for (const auto& e : s) {view(e);cerr << " ";} cerr << endl;}
template<typename T>void view(const multiset<T>& s) {for (const auto& e : s) {view(e);cerr << " ";} cerr << endl;}
template<typename T>void view(const unordered_set<T>& s) {for (const auto& e : s) {view(e);cerr << " ";} cerr << endl;}
template<typename T1, typename T2>void view(const map<T1, T2>& mp){for (const auto& e : mp) {view(e);cerr << " ";} cerr << endl;}

struct lowlink {
    const vvi &g;
    vi parent, ord, low, ap, connected_comp_diff;
    vc<pair<int, int>> bridge;

    lowlink(const vvi &g_) : g(g_){
        parent.resize(len(g), -2);
        ord.resize(len(g), 0);
        low.resize(len(g), 0);
        connected_comp_diff.resize(len(g), 0);
        int k = 0;
        rep(v, len(g)) if (parent[v] == -2){
            parent[v] = -1;
            k = dfs(v, k);
        }
    }

    int dfs(int v, int k){
        bool flag = false;
        ord[v] = k++;
        low[v] = ord[v];
        int child = 0;
        for (auto u : g[v]){
            if (parent[u] == -2){
                child++;
                parent[u] = v;
                k = dfs(u, k);
                low[v] = min(low[v], low[u]);
                if (parent[v] != -1 && ord[v] <= low[u]){
                    connected_comp_diff[v]++;
                    flag = true;
                }
                if (ord[v] < low[u]) bridge.push_back({v, u});
            }else if (u != parent[v]){
                low[v] = min(low[v], ord[u]);
            }
        }
        if (parent[v] == -1) connected_comp_diff[v] = child - 1;
        if ((parent[v] == -1 && len(g[v]) >= 2) || flag) ap.push_back(v);
        return k;
    }
};

void solve(){
    int N, M, s, t; cin >> N >> M >> s >> t; s--; t--;
    vvi g(N);
    rep(i, M){
        int a, b; cin >> a >> b; a--;b--;
        g[a].push_back(b);
        g[b].push_back(a);
    }
    lowlink link(g);
    set<pair<int, int>> bridges;
    for (auto [a, b] : link.bridge) bridges.insert(minmax(a, b));
    vc<pair<int, int>> dist(N, {inf, 0});
    priority_queue<pair<pair<int, int>, int>, vc<pair<pair<int, int>, int>>, greater<>> pq;
    pq.push({{0, 0}, s});
    while (!pq.empty()){
        auto [d, v] = pq.top(); pq.pop();
        if (dist[v] < d) continue;
        for (auto u : g[v]){
            auto nd = d;
            nd.first++;
            if (!bridges.count(minmax(u, v))) nd.second--;
            if (dist[u] > nd){
                dist[u] = nd;
                pq.push({dist[u], u});
            }
        }
    }
    if (dist[t].first == inf) cout << -1 << endl;
    else cout << -dist[t].second << endl;
}


int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int T = 1;
    // cin >> T;
    while (T--) solve();
}