//Giorgi Kldiashvili

#include <bits/stdc++.h>

#define ll long long
#define M 1234567LL

using namespace std;

priority_queue < pair < int, int > > q;
int n, m, st, en;
vector < pair < int, int > > G[200020];
int D[200020];

void go(int v)
{
  for(int i = 0; i < G[v].size(); ++ i)
  {
    int to = G[v][i].first;
    if(D[to] > D[v] + G[v][i].second)
    {
      D[to] = D[v] + G[v][i].second;
      q.push(make_pair(- D[to], to));
    }
  }
  while(!q.empty())
  {
    int x = q.top().second;
    int y = -q.top().first;
    q.pop();
    if(D[x] == y)
      go(x);
  }
}

int main()
{
  scanf("%d %d %d %d", &n, &m, &st, &en);

  for(int i = 0; i < m; ++ i)
  {
    int x, y;
    scanf("%d %d", &x, &y);
    G[x].push_back(make_pair(y, 0));
    G[y].push_back(make_pair(x, 1));
  }
  for(int i = 1; i <= n; ++ i)
    D[i] = 1e7;
  D[st] = 0;
  go(st);

  if(D[en] == 1e7)
    printf("X\n");
  else
    printf("%d\n", D[en]);
}