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();
        TreeMap<Integer, HashSet<Integer>> map = new TreeMap<>();
        for (int i = 0; i < m; i++) {
            int x = sc.nextInt();
            int y = sc.nextInt();
            if (!map.containsKey(x)) {
                map.put(x, new HashSet<>());
            }
            map.get(x).add(y);
        }
        HashSet<Integer> enable = new HashSet<>();
        enable.add(n);
        for (HashSet<Integer> exist : map.values()) {
            HashSet<Integer> next = new HashSet<>();
            for (int x : exist) {
                if (enable.contains(x - 1) || enable.contains(x + 1)) {
                    next.add(x);
                }
            }
            for (int x : exist) {
                enable.remove(x);
            }
            enable.addAll(next);
        }
        System.out.println(enable.size());
    }
}
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 String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}