#include<iostream>
#include<vector>
#include<queue>
using namespace std;
int N,M,A,B;
vector<int>G[1<<17];
int vis[1<<17];
main()
{
	cin>>N>>M>>A>>B;
	for(int i=0;i<=N;i++)vis[i]=1e9;
	A--;
	for(;M--;)
	{
		int l,r;cin>>l>>r;
		l--;
		G[l].push_back(r);
		G[r].push_back(l);
	}
	queue<int>P;
	for(int i=0;i<=A;i++)
	{
		P.push(i);
		vis[i]=0;
	}
	while(!P.empty())
	{
		int u=P.front();P.pop();
		for(int v:G[u])if(vis[v]>vis[u]+1)
		{
			vis[v]=vis[u]+1;
			P.push(v);
		}
	}
	int ans=1e9;
	for(int i=B;i<=N;i++)if(ans>vis[i])ans=vis[i];
	cout<<(ans==(int)1e9?-1:ans)<<endl;
}