import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const M = readInt();
      const A = readInt() - 1;
      const B = readInt();
      auto L = new int[M];
      auto R = new int[M];
      foreach (i; 0 .. M) {
        L[i] = readInt() - 1;
        R[i] = readInt();
      }
      
      auto G = new int[][N + 1];
      foreach (i; 0 .. M) {
        G[L[i]] ~= i;
        G[R[i]] ~= i;
      }
      
      DList!int que;
      auto dist = new int[N + 1];
      dist[] = -1;
      foreach (u; 0 .. A + 1) {
        dist[u] = 0;
        que ~= u;
      }
      for (; !que.empty; ) {
        const u = que.front;
        que.removeFront;
        foreach (i; G[u]) {
          const v = L[i] ^ R[i] ^ u;
          if (dist[v] == -1) {
            dist[v] = dist[u] + 1;
            que ~= v;
          }
        }
      }
      
      int ans = -1;
      foreach (u; B .. N + 1) {
        if (dist[u] != -1) {
          if (ans == -1 || ans > dist[u]) {
            ans = dist[u];
          }
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}