#include <iostream>
#include <numeric>
#include <vector>
#include <queue>

template <class Cost = int>
struct Edge {
    int src, dst;
    Cost cost;

    Edge() = default;
    Edge(int src, int dst, Cost cost = 1)
        : src(src), dst(dst), cost(cost){};

    bool operator<(const Edge<Cost>& e) const { return cost < e.cost; }
    bool operator>(const Edge<Cost>& e) const { return cost > e.cost; }
};

template <class Cost = int>
struct Graph : public std::vector<std::vector<Edge<Cost>>> {
    Graph(int n = 0) : std::vector<std::vector<Edge<Cost>>>(n) {}

    void span(bool direct, int src, int dst, Cost cost = 1) {
        (*this)[src].emplace_back(src, dst, cost);
        if (!direct) (*this)[dst].emplace_back(dst, src, cost);
    }
};

template <class Cost>
std::vector<Cost> bfs(const Graph<Cost>& graph, std::vector<int> ss) {
    std::vector<Cost> dist(graph.size(), -1);
    std::queue<int> que;
    for (auto s : ss) {
        dist[s] = 0;
        que.push(s);
    }

    while (!que.empty()) {
        auto v = que.front();
        que.pop();

        for (const auto& e : graph[v]) {
            if (dist[e.dst] != -1) continue;
            dist[e.dst] = dist[v] + e.cost;
            que.push(e.dst);
        }
    }

    return dist;
}

void solve() {
    int n, m, s, t;
    std::cin >> n >> m >> s >> t;

    Graph<> graph(n + 1);
    while (m--) {
        int l, r;
        std::cin >> l >> r;
        graph.span(false, --l, r);
    }

    std::vector<int> ss(s);
    std::iota(ss.begin(), ss.end(), 0);

    auto ds = bfs(graph, ss);
    int ans = -1;
    for (int v = t; v <= n; ++v) {
        if (ds[v] == -1) continue;
        if (ans == -1 || ds[v] < ans) ans = ds[v];
    }
    std::cout << ans << "\n";
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}