#if __has_include("../library/Basic/Debug.hpp")

#include "../library/Basic/Debug.hpp"

#else

/* ----- Header Files ----- */
// IO
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm
#include <algorithm>
#include <cmath>
#include <numeric>

// container
#include <vector>
#include <string>
#include <tuple>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>

// others
#include <random>
#include <limits>
#include <functional>
#include <ctime>
#include <cassert>
#include <cstdint>


/* ----- Type Alias ----- */
using Bool = bool;
using Int = long long int;
using Real = long double;
using Char = char;
using String = std::string;
template <class... Ts>
using Tuple = std::tuple<Ts...>;

template <class T>
using Vector = std::vector<T>;
template <class T>
using Matrix = Vector<Vector<T>>;
template <class T>
using Queue = std::queue<T>;
template <class T>
using Stack = std::stack<T>;
template <class T>
using Deque = std::deque<T>;

template <class T>
using MaxHeap = std::priority_queue<T>;
template <class T>
using MinHeap = std::priority_queue<T, std::vector<T>, std::greater<T>>;
template <class T>
using Set = std::set<T>;
template <class T, class U>
using Map = std::map<T, U>;

template <class T, class... Us>
using Func = std::function<T(Us...)>;

template <class T>
T genv(T v) { return v; }

template <class T, class... Ts>
auto genv(size_t l, Ts... ts) {
    return Vector<decltype(genv<T>(ts...))>(l, genv<T>(ts...));
}

template <class Cost = Int>
struct Edge {
    Int src, dst;
    Cost cost;
    Edge(Int src = -1, Int dst = -1, Cost cost = 1)
        : src(src), dst(dst), cost(cost){};

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

template <class Cost = Int>
using Edges = Vector<Edge<Cost>>;
template <class Cost = Int>
using Graph = Vector<Vector<Edge<Cost>>>;

#endif

/* ----- Misc ----- */
void fastio() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
}

struct Fout {
    Int precision;
    Fout(Int precision) : precision(precision) {}
};
std::ostream& operator<<(std::ostream& os, const Fout& fio) {
    os << std::fixed << std::setprecision(fio.precision);
    return os;
}


/* ----- Constants ----- */
constexpr Int P = 1000000;
constexpr Int INF = std::numeric_limits<Int>::max() / 3;
// constexpr Int MOD = 1000000007;
// constexpr Real PI = acos(-1);
// constexpr Real EPS = 1e-10;
// std::mt19937 mt(int(std::time(nullptr)));

int main() {
    Vector<Bool> isprime(P, true);
    isprime[0] = isprime[1] = false;

    for (Int p = 2; p * p < P; ++p) {
        for (Int q = p; p * q < P; ++q) {
            isprime[p * q] = false;
        }
    }

    Int n, m, s, g;
    std::cin >> n >> m >> s >> g;
    --s, --g;

    Graph<> graph(n * 2);
    for (Int i = 0; i < m; ++i) {
        Int u, v;
        std::cin >> u >> v;
        --u, --v;

        graph[u].emplace_back(u, v + n);
        graph[v].emplace_back(v, u + n);
        graph[u + n].emplace_back(u + n, v);
        graph[v + n].emplace_back(v + n, u);
    }

    Vector<Int> dist(n * 2, INF);
    dist[s] = 0;
    Queue<Int> que;
    que.push(s);

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

        for (auto e : graph[v]) {
            if (dist[e.dst] < INF) continue;
            dist[e.dst] = dist[e.src] + 1;
            que.push(e.dst);
        }
    }

    Int ans = -1;
    if (dist[g] == 2) {
        ans = 2;
    } else if (dist[g + n] < INF) {
        for (ans = dist[g + n]; !isprime[ans]; ans += 2) {}
    }

    std::cout << ans << std::endl;
    return 0;
}



Measured memory_bytes: 22288