/* -*- coding: utf-8 -*-
 *
 * 1244.cc:  No.1244 Black Segment - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100000;

/* typedef */

typedef vector<int> vi;
typedef queue<int> qi;

/* global variables */

vi nbrs[MAX_N];
int ds[MAX_N + 1];

/* subroutines */

/* main */

int main() {
  int n, m, a, b;
  scanf("%d%d%d%d", &n, &m, &a, &b);
  a--;

  memset(ds, -1, sizeof(ds));
  qi q;

  for (int i = 0; i < m; i++) {
    int l, r;
    scanf("%d%d", &l, &r);
    l--;
    nbrs[l].push_back(r);
    nbrs[r].push_back(l);
    if (l <= a) ds[l] = 0, q.push(l);
  }

  int gl = -1;
  while (! q.empty()) {
    int u = q.front(); q.pop();
    int ud = ds[u], vd = ud + 1;
    if (u >= b) {
      gl = ud;
      break;
    }

    vi &nbru = nbrs[u];
    for (vi::iterator vit = nbru.begin(); vit != nbru.end(); vit++) {
      int v = *vit;
      if (ds[v] < 0) {
	ds[v] = vd;
	q.push(v);
      }
    }
  }

  printf("%d\n", gl);
  return 0;
}