import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int a = sc.nextInt();
        int b = sc.nextInt();
        ArrayList<ArrayList<Integer>> graph = new ArrayList<>();
        for (int i = 0; i <= n + 1; i++) {
            graph.add(new ArrayList<>());
        }
        for (int i = 0; i < m; i++) {
            int x = sc.nextInt();
            int y = sc.nextInt() + 1;
            graph.get(x).add(y);
            graph.get(y).add(x);
        }
        PriorityQueue<Path> queue = new PriorityQueue<>();
        for (int i = 1; i <= a; i++) {
            queue.add(new Path(i, 0));
        }
        int[] costs = new int[n + 2];
        Arrays.fill(costs, Integer.MAX_VALUE);
        while (queue.size() > 0) {
            Path p = queue.poll();
            if (costs[p.idx] <= p.value) {
                continue;
            }
            costs[p.idx] = p.value;
            for (int x : graph.get(p.idx)) {
                queue.add(new Path(x, p.value + 1));
            }
        }
        int min = Integer.MAX_VALUE;
        for (int i = b + 1; i < costs.length; i++) {
            min = Math.min(min, costs[i]);
        }
        if (min == Integer.MAX_VALUE) {
            System.out.println(-1);
        } else {
            System.out.println(min);
        }
    }
    
    static class Path implements Comparable<Path> {
        int idx;
        int value;
        
        public Path(int idx, int value) {
            this.idx = idx;
            this.value = value;
        }
        
        
        public int compareTo(Path another) {
            return value - another.value;
        }
    }
}