// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
#define int ll
#define double ld
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define rep1(i,n) for (int i = 1; i <= (int)(n); i++)
#define repR(i,n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i,n) for (int i = (int)(n); i >= 1; i--)
#define loop(i,a,B) for (int i = a; i B; i++)
#define loopR(i,a,B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
template <class Int> auto constexpr inf = numeric_limits<Int>::max()/2-1;
auto constexpr INF32 = inf<int32_t>;
auto constexpr INF64 = inf<int64_t>;
auto constexpr INF   = inf<int>;
#ifdef LOCAL
#include "debug.hpp"
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T> >;
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
ostream& operator<<(ostream& os, T const& v) { bool f = true; for (auto const& x : v) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
istream& operator>>(istream& is, T &v) { for (auto& x : v) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T,S> const& p) { return os << "(" << p.first << ", " << p.second << ")"; }
template <class T, class S> istream& operator>>(istream& is, pair<T,S>& p) { return is >> p.first >> p.second; }
struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup;
template <class F> struct FixPoint : private F {
    constexpr FixPoint(F&& f) : F(forward<F>(f)) {}
    template <class... T> constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward<T>(x)...); }
};
struct MakeFixPoint {
    template <class F> constexpr auto operator|(F&& f) const { return FixPoint<F>(forward<F>(f)); }
};
#define MFP MakeFixPoint()|
#define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T,d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T,d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T,0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d = 1> using vec = typename vec_impl<T,d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args&&... args) { return vec_impl<T,d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T, class U> constexpr bool chmin(T& x, U const& y) { if (x > y) { x = y; return true; } return false; }
template <class T, class U> constexpr bool chmax(T& x, U const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b,e,typename iterator_traits<It>::value_type{}); }
template <class T> int sz(T const& x) { return x.size(); }
template <class C, class T> int lbd(C const& v, T const& x) {
    return lower_bound(v.begin(), v.end(), x)-v.begin();
}
template <class C, class T> int ubd(C const& v, T const& x) {
    return upper_bound(v.begin(), v.end(), x)-v.begin();
}
const int dx[] = { 1,0,-1,0 };
const int dy[] = { 0,1,0,-1 };
constexpr int popcnt(ll x) { return __builtin_popcountll(x); }
template <class Int> struct Random {
    mt19937_64 mt{random_device{}()};
    //mt19937_64 mt{(unsigned)time(0)};
    Int a,b; // [a,b]
    Random(Int a, Int b) : a(a), b(b) {}
    Int operator()() { return uniform_int_distribution<Int>(a,b)(mt); }
};
template <class Int> Int rand(Int a, Int b) { // [a,b]
    static mt19937_64 mt{random_device{}()};
    return uniform_int_distribution<Int>(a,b)(mt);
}
// <<<
// >>> sieve

namespace Sieve {
    constexpr int MAX = 1e6;
    vector<int> ps, pf, mu; // primes, min prime factor, moebius
    auto sieve_init = [](){
        pf.resize(MAX+1);
        iota(pf.begin(), pf.end(), 0);
        mu.resize(MAX+1,-1);
        mu[1] = 1;
        for (int i = 2; i <= MAX; ++i) {
            if (pf[i] == i) ps.push_back(i);
            for (int p : ps) {
                const int x = p*i;
                if (p > pf[i] || x > MAX) break;
                pf[x] = p;
                mu[x] = -mu[i];
                if (i%p == 0) mu[x] = 0;
            }
        }
        return 0;
    }();
    bool is_prime(int n) {
        assert(0 <= n); assert(n <= MAX);
        return pf[n] == n && n >= 2;
    }
    vector<pair<int,int> > prime_factor(int n) {
        assert(0 <= n); assert(n <= MAX);
        vector<pair<int,int> > ret;
        while (n > 1) {
            int p = pf[n], i = 0;
            while (pf[n] == p) ++i, n /= p;
            ret.emplace_back(p,i);
        }
        return ret;
    }
    vector<int> divisors(int n) {
        assert(0 <= n); assert(n <= MAX);
        vector<int> ret = {1};
        for (auto p : prime_factor(n)) {
            int m = ret.size();
            for (int i = 0; i < m; ++i) {
                for (int j = 0, v = 1;  j < p.second; ++j) {
                    v *= p.first;
                    ret.push_back(ret[i]*v);
                }
            }
        }
        return ret;
    }
}
using namespace Sieve;

// <<<

int32_t main() {
    int n,m,s,t; cin >> n >> m >> s >> t; --s,--t;
    vector<vector<int>> g(n);
    rep (_,m) {
        int a,b; cin >> a >> b; --a,--b;
        g[a].eb(b);
        g[b].eb(a);
    }

    auto dist = make_v<int,2>(2,n,INF);
    queue<pair<int,int>> q;
    dist[0][s] = 0;
    q.emplace(0,s);
    while (q.size()) {
        int p,x; tie(p,x) = q.front(); q.pop();
        for (int y : g[x]) {
            if (chmin(dist[p^1][y], dist[p][x] + 1)) {
                q.emplace(p^1,y);
            }
        }
    }

    int ans = INF;
    if (dist[0][t] == 2) ans = 2;
    if (dist[1][t] < INF) {
        for (int d = dist[1][t]; ; d += 2) {
            if (is_prime(d)) {
                chmin(ans, d);
                break;
            }
        }
    }

    cout << (ans < INF ? ans : -1) << endl;

}



Measured memory_bytes: 27120