#include <bits/stdc++.h>
using namespace std;
using lint = long long;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios {
    fast_ios() { cin.tie(nullptr), ios::sync_with_stdio(false), cout << fixed << setprecision(20); };
} fast_ios_;
#define FOR(i, begin, end) for (int i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end) for (int i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define ALL(x) (x).begin(), (x).end()
//
template <typename T, typename V>
void ndarray(vector<T>& vec, const V& val, int len) { vec.assign(len, val); }
template <typename T, typename V, typename... Args>
void ndarray(vector<T>& vec, const V& val, int len, Args... args)
{
    vec.resize(len), for_each(begin(vec), end(vec), [&](T& v) { ndarray(v, val, args...); });
}
template <typename T>
bool chmax(T& m, const T q) { return m < q ? (m = q, true) : false; }
template <typename T>
bool chmin(T& m, const T q) { return m > q ? (m = q, true) : false; }
template <typename T1, typename T2>
pair<T1, T2> operator+(const pair<T1, T2>& l, const pair<T1, T2>& r) { return make_pair(l.first + r.first, l.second + r.second); }
template <typename T1, typename T2>
pair<T1, T2> operator-(const pair<T1, T2>& l, const pair<T1, T2>& r) { return make_pair(l.first - r.first, l.second - r.second); }
template <typename T>
vector<T> srtunq(vector<T> vec) { return sort(vec.begin(), vec.end()), vec.erase(unique(vec.begin(), vec.end()), vec.end()), vec; }
template <typename T>
istream& operator>>(istream& is, vector<T>& vec)
{
    return for_each(begin(vec), end(vec), [&](T& v) { is >> v; }), is;
}

// output
template <typename T, typename V>
ostream& dmpseq(ostream&, const T&, const string&, const string&, const string&);
#if __cplusplus >= 201703L
template <typename... T>
ostream& operator<<(ostream& os, const tuple<T...>& tpl)
{
    return apply([&os](auto&&... args) { ((os << args << ','), ...); }, tpl), os;
}
#endif
//
template <typename T1, typename T2>
ostream& operator<<(ostream& os, const pair<T1, T2>& p) { return os << '(' << p.first << ',' << p.second << ')'; }
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& x) { return dmpseq<vector<T>, T>(os, x, "[", ",", "]"); }
template <typename T>
ostream& operator<<(ostream& os, const deque<T>& x) { return dmpseq<deque<T>, T>(os, x, "deq[", ",", "]"); }
template <typename T>
ostream& operator<<(ostream& os, const set<T>& x) { return dmpseq<set<T>, T>(os, x, "{", ",", "}"); }
template <typename T, typename TH>
ostream& operator<<(ostream& os, const unordered_set<T, TH>& x) { return dmpseq<unordered_set<T, TH>, T>(os, x, "{", ",", "}"); }
template <typename T>
ostream& operator<<(ostream& os, const multiset<T>& x) { return dmpseq<multiset<T>, T>(os, x, "{", ",", "}"); }
template <typename TK, typename T>
ostream& operator<<(ostream& os, const map<TK, T>& x) { return dmpseq<map<TK, T>, pair<TK, T>>(os, x, "{", ",", "}"); }
template <typename TK, typename T, typename TH>
ostream& operator<<(ostream& os, const unordered_map<TK, T, TH>& x) { return dmpseq<unordered_map<TK, T, TH>, pair<TK, T>>(os, x, "{", ",", "}"); }
template <typename T, typename V>
ostream& dmpseq(ostream& os, const T& seq, const string& pre, const string& sp, const string& suf)
{
    return os << pre, for_each(begin(seq), end(seq), [&](V x) { os << x << sp; }), os << suf;
}
template <typename T>
void print(const vector<T>& x) { dmpseq<vector<T>, T>(cout, x, "", " ", "\n"); }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl

template<typename T>
struct ShortestPath
{
    int V, E;
    int INVALID = -1;
    std::vector<std::vector<std::pair<int, T>>> to;
    ShortestPath() = default;
    ShortestPath(int V) : V(V), E(0), to(V) {}
    void add_edge(int s, int t, T len) {
        assert(0 <= s and s < V);
        assert(0 <= t and t < V);
        to[s].emplace_back(t, len);
        E++;
    }

    std::vector<T> dist;
    std::vector<int> prev;
    // Dijkstra algorithm
    // Complexity: O(E log E)
    void Dijkstra(int s) {
        assert(0 <= s and s < V);
        dist.assign(V, std::numeric_limits<T>::max());
        dist[s] = 0;
        prev.assign(V, INVALID);
        using P = std::pair<T, int>;
        std::priority_queue<P, std::vector<P>, std::greater<P>> pq;
        pq.emplace(0, s);
        while(!pq.empty()) {
            T d;
            int v;
            std::tie(d, v) = pq.top();
            pq.pop();
            if (dist[v] < d) continue;
            for (auto nx : to[v]) {
                T dnx = d + nx.second;
                if (dist[nx.first] > dnx) {
                    dist[nx.first] = dnx, prev[nx.first] = v;
                    pq.emplace(dnx, nx.first);
                }
            }
        }
    }

    // Bellman-Ford algorithm
    // Complexity: O(VE)
    bool BellmanFord(int s, int nb_loop) {
        assert(0 <= s and s < V);
        dist.assign(V, std::numeric_limits<T>::max());
        dist[s] = 0;
        prev.assign(V, INVALID);
        for (int l = 0; l < nb_loop; l++) {
            bool upd = false;
            for (int v = 0; v < V; v++) {
                if (dist[v] == std::numeric_limits<T>::max()) continue;
                for (auto nx : to[v]) {
                    T dnx = dist[v] + nx.second;
                    if (dist[nx.first] > dnx) {
                        dist[nx.first] = dnx, prev[nx.first] = v;
                        upd = true;
                    }
                }
            }
            if (!upd) return true;
        }
        return false;
    }
    // Warshall-Floyd algorithm
    // Complexity: O(E + V^3)
    std::vector<std::vector<T>> dist2d;
    void WarshallFloyd() {
        dist2d.assign(V, std::vector<T>(V, std::numeric_limits<T>::max()));
        for (int i = 0; i < V; i++) {
            dist2d[i][i] = 0;
            for (auto p : to[i]) dist2d[i][p.first] = min(dist2d[i][p.first], p.second);
        }
        for (int k = 0; k < V; k++) {
            for (int i = 0; i < V; i++) {
                if (dist2d[i][k] = std::numeric_limits<T>::max()) continue;
                for (int j = 0; j < V; j++) {
                    if (dist2d[k][j] = std::numeric_limits<T>::max()) continue;
                    dist2d[i][j] = min(dist2d[i][j], dist2d[i][k] + dist2d[k][j]);
                }
            }
        }
    }
};

int main()
{
    int N, M, A, B;
    cin >> N >> M >> A >> B;
    A--;
    vector<pint> LR(M);
    for (auto &p : LR)
    {
        cin >> p.first >> p.second;
        p.first--;
    }

    int V = N + 2;
    ShortestPath<int> graph(V);
    for (auto [l, r] : LR)
    {
        graph.add_edge(l, r, 1);
        graph.add_edge(r, l, 1);
    }
    REP(i, A - 1) graph.add_edge(A, i, 0);
    FOR(j, B, V - 1) graph.add_edge(j + 1, B, 0);
    // dbg(A);
    // dbg(B);
    // dbg(LR);
    graph.Dijkstra(A);
    auto ret = graph.dist[B];
    cout << (ret > V + 10 ? -1 : ret) << '\n';
}