int n,m,a,b,x,y,l[1d5],r[1d5],c[1d5];
graph g;
deque<int> q;
{
	rd(n,m,a,b,(l--,r)(m));
	g.setEdge(n+1,m,l,r);
	rep(i,a){
		c[i]=1;
		q.push_back(i);
	}
	while(!q.empty()){
		x=q.front();
		q.pop_front();
		if(x>=b){
			wt(c[x]-1);
			exit(0);
		}
		rep[g.edge[x]](y,g.es[x]){
			if(!c[y]){
				c[y]=c[x]+1;
				q.push_back(y);
			}
		}
	}
	wt(-1);
}