#include <bits/stdc++.h>
using namespace std;
int n,m,s,e;
bool g[51][51]={0};
int t[51]={0};
queue<int> q;
int main()
{
    freopen("city.in","r",stdin);
    freopen("city.out","w",stdout);
      cin>>n>>m>>s>>e;
      for(int i=1;i<=m;i++)
      {
          int z,k;
          cin>>z>>k;
          g[z][k]=1;
      } 
      q.push(s);t[s]=1;
      while(!q.empty())
      {
          int now=q.front();q.pop();
          if(now==e)
          {
          cout<<t[now]<<endl;
          return 0;
          }
          for(int k=1;k<=n;k++)
          {
              if(g[now][k]&&t[k]==0)
              {//cout<<now<<' '<<k<<endl;
                  t[k]=t[now]+1;
                  q.push(k);
              }
              
          }
      }
      
      
}