import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int m = sc.nextInt();
        int[] lefts = new int[m];
        int[] rights = new int[m];
        TreeMap<Integer, Integer> compress = new TreeMap<>();
        for (int i = 0; i < m; i++) {
            lefts[i] = sc.nextInt();
            compress.put(lefts[i], null);
            rights[i] = sc.nextInt();
            compress.put(rights[i], null);
        }
        compress.put(1, null);
        compress.put(n, null);
        int idx = 0;
        ArrayList<Integer> distance = new ArrayList<>();
        int prev = -1;
        for (int x : compress.keySet()) {
            compress.put(x, idx++);
            distance.add(x - prev);
            prev = x;
        }
        ArrayList<ArrayList<Path>> graph = new ArrayList<>();
        for (int i = 0; i < idx; i++) {
            graph.add(new ArrayList<>());
            if (i > 0) {
                graph.get(i - 1).add(new Path(i, distance.get(i) * 2));
            }
        }
        for (int i = 0; i < m; i++) {
            graph.get(compress.get(lefts[i])).add(new Path(compress.get(rights[i]), 2 * (rights[i] - lefts[i]) - 1));
        }
        int[] costs = new int[idx];
        Arrays.fill(costs, Integer.MAX_VALUE);
        PriorityQueue<Path> queue = new PriorityQueue<>();
        queue.add(new Path(0, 0));
        while (queue.size() > 0) {
            Path p = queue.poll();
            if (costs[p.idx] <= p.value) {
                continue;
            }
            costs[p.idx] = p.value;
            for (Path x : graph.get(p.idx)) {
                queue.add(new Path(x.idx, x.value + p.value));
            }
        }
        System.out.println(costs[idx - 1]);
    }
    
    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;
        }
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}