import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int[] a = new int[10001];
        int l,m,sum = 0;
        Scanner scanner = new Scanner(System.in);
        l = scanner.nextInt();
        m = scanner.nextInt();
        for (int i = 0; i <= l; i++) {
            a[i] = 1;
        }
        for (int i = 0; i < m; i++) {
            int u = scanner.nextInt();
            int v = scanner.nextInt();
            for (int j = u; j <= v; j++) {
                a[j] = 0;
            }
        }
        for (int i = 0; i <= l; i++) {
            if(a[i] == 1){
                sum++;
            }
        }
        System.out.println(sum);
    }
}