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();
        List<List<Integer>> graph = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            graph.add(new ArrayList<>());
        }
        for (int i = 0; i < m; i++) {
            graph.get(sc.nextInt() - 1).add(sc.nextInt() - 1);
        }
        int[] starts = new int[]{0, n - 2, n - 1};
        Deque<Path> deq = new ArrayDeque<>();
        int[][] costs = new int[3][n];
        for (int i = 0; i < 3; i++) {
            Arrays.fill(costs[i], Integer.MAX_VALUE / 3);
            deq.add(new Path(starts[i], 0));
            while (deq.size() > 0) {
                Path p = deq.poll();
                if (costs[i][p.idx] <= p.value) {
                    continue;
                }
                costs[i][p.idx] = p.value;
                for (int x : graph.get(p.idx)) {
                    deq.add(new Path(x, p.value + 1));
                }
            }
        }
        int ans = Math.min(costs[0][n - 2] + costs[1][n - 1] + costs[2][0]
            , costs[0][n - 1] + costs[2][n - 2] + costs[1][0]);
        System.out.println(ans >= Integer.MAX_VALUE / 3 ? -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();
        }
    }
    
}