#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int N,M,A,B;
vector<int> E[101010];
int D[101010];

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>M>>A>>B;
	FOR(i,N+1) D[i]=1<<20;
	A--;
	FOR(i,M) {
		cin>>x>>y;
		E[x-1].push_back(y);
		E[y].push_back(x-1);
	}
	
	queue<int> Q;
	for(i=0;i<=A;i++) {
		D[i]=0;
		Q.push(i);
	}
	while(Q.size()) {
		x=Q.front();
		Q.pop();
		if(x>=B) return _P("%d\n",D[x]);
		FORR(e,E[x]) if(D[e]>D[x]+1) {
			D[e]=D[x]+1;
			Q.push(e);
		}
	}
	cout<<-1<<endl;
	
	
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}