import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		//List<Integer> list=new ArrayList<Integer>();
		//Integer[] array = new Integer[m];
        int n = Integer.parseInt(sc.next());
        int m = Integer.parseInt(sc.next());
        int[] ary = new int[2];
        ary[0] = 1;
        ary[1] = n;
        for (int i = 0; i < m; i++) {
            int l = Integer.parseInt(sc.next());
            int r = Integer.parseInt(sc.next());
            if (ary[1] < l || ary[0] > r) {
                System.out.println(0);
                return;
            }
            if (ary[0] < l) {
                ary[0] = l;
            }
            if (ary[1] > r) {
                ary[1] = r;
            }
        }
        System.out.println(ary[1] - ary[0] + 1);
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz