#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> p_ll;

template<class T>
void debug(T itr1, T itr2) { auto now = itr1; while(now<itr2) { cout << *now << " "; now++; } cout << endl; }
#define repr(i,from,to) for (ll i=(ll)from; i<(ll)to; i++)
#define all(vec) vec.begin(), vec.end()
#define rep(i,N) repr(i,0,N)
#define per(i,N) for (int i=(int)N-1; i>=0; i--)

const ll MOD = pow(10,9)+7;
const ll LLINF = pow(2,61)-1;
const int INF = pow(2,30)-1;

vector<ll> fac;
void c_fac(int x=pow(10,6)+10) { fac.resize(x,true); rep(i,x) fac[i] = i ? (fac[i-1]*i)%MOD : 1; }
ll inv(ll a, ll m=MOD) { ll b = m, x = 1, y = 0; while (b!=0) { int d = a/b; a -= b*d; swap(a,b); x -= y*d; swap(x,y); } return (x+m)%m; }
ll nck(ll n, ll k) { return fac[n]*inv(fac[k]*fac[n-k]%MOD)%MOD; }
ll gcd(ll a, ll b) { if (a<b) swap(a,b); return b==0 ? a : gcd(b, a%b); }
ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }

struct button { ll l, r; };

// ----------------------------------------------------------------------
// ----------------------------------------------------------------------

struct dijkstra {

  ll N;
  struct edge { ll to, d; };
  vector<vector<edge>> adj;
  vector<ll> minlen;

  dijkstra(ll n) { N = n; adj.resize(N); minlen.assign(N,LLINF); }
  ll operator[](const ll &x) const { return minlen[x]; }

  void add_edge(ll from, ll to, ll d){
    adj[from].push_back({to,d});
    adj[to].push_back({from,d});
  }
  vector<ll> calc(ll n=0) {
    auto c = [](const p_ll &x, const p_ll &y){return x.second>y.second;};
    priority_queue<p_ll, vector<p_ll>, decltype(c)> q(c);
    minlen[n] = 0; q.push(make_pair(n,0));
    while(q.size()) {
      p_ll now = q.top(); q.pop();
      ll np = now.first, nd = now.second;
      if (nd>minlen[np]) continue;
      for (auto x: adj[np]) {
        if (minlen[x.to]<=minlen[np]+x.d) continue;
        q.push(make_pair(x.to, minlen[np]+x.d));
        minlen[x.to] = minlen[np]+x.d;
      }
    }
    return minlen;
  }

};

// ----------------------------------------------------------------------
// ----------------------------------------------------------------------

int main() {
  ll N, M, A, B; cin >> N >> M >> A >> B; A--;
  button b[M]; rep(i,M) { cin >> b[i].l >> b[i].r; b[i].l--; }
  map<ll, ll> m; rep(i,M) { m[b[i].l]++; m[b[i].r]++; }
  ll tm = 1; for (auto &x: m) { x.second = tm; tm++; }
  // for (auto x: m) cout << x.first << "-" << x.second << " "; cout << endl;

  N = m.size() + 2;
  dijkstra d(N); rep(i,M) d.add_edge(m[b[i].l],m[b[i].r],1);
  for (auto x: m) { 
    if (x.first<=A) d.add_edge(0,x.second,0);
    if (x.first>=B) d.add_edge(N-1,x.second,0);
  }

  vector<ll> dis = d.calc();
  // debug(all(dis));
  ll result = dis[N-1]==LLINF ? -1 : dis[N-1];
  cout << result << endl;
  return 0;
}