import java.io.*;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int m = sc.nextInt();
        int a = sc.nextInt() - 1;
        int b = sc.nextInt() - 1;
        List<List<Integer>> graph = new ArrayList<>();
        for (int i = 0; i <= n; i++) {
            graph.add(new ArrayList<>());
        }
        for (int i = 0; i < m; i++) {
            int p = sc.nextInt() - 1;
            int q = sc.nextInt();
            graph.get(p).add(q);
            graph.get(q).add(p);
        }
        int[] costs = new int[n + 1];
        Arrays.fill(costs, Integer.MAX_VALUE);
        Deque<Path> deq = new ArrayDeque<>();
        for (int i = 0; i <= a; i++) {
            deq.add(new Path(i, 0));
        }
        while (deq.size() > 0) {
            Path p = deq.poll();
            if (costs[p.idx] <= p.value) {
                continue;
            }
            costs[p.idx] = p.value;
            for (int x : graph.get(p.idx)) {
                deq.add(new Path(x, p.value + 1));
            }
        }
        int ans = Integer.MAX_VALUE;
        for (int i = b + 1; i <= n; i++) {
            ans = Math.min(ans, costs[i]);
        }
        System.out.println(ans == Integer.MAX_VALUE ? -1 : ans);
    }
    
    static class Path {
        int idx;
        int value;
        
        public Path(int idx, int value) {
            this.idx = idx;
            this.value = value;
        }
    }
}
class Scanner {
    BufferedReader br;
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
        } catch (Exception e) {
            
        }
    }
    
    public int nextInt() {
        return Integer.parseInt(next());
    }
    
    public long nextLong() {
        return Long.parseLong(next());
    }
    
    public double nextDouble() {
        return Double.parseDouble(next());
    }
    
    public String next() {
        try {
            while (!st.hasMoreTokens()) {
                st = new StringTokenizer(br.readLine());
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            return st.nextToken();
        }
    }
    
}