import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		int N = 0;
		int M = 0;
		String s;
		Scanner sc = new Scanner(System.in);
		N = Integer.parseInt(sc.next());
		M = Integer.parseInt(sc.next());
		int i;
		int temp;
		int max = 100000, min = 0;
		for (i = 0; i < M; i++) {
			temp = Integer.parseInt(sc.next());
			if (min < temp) {
				min = temp;
			}
			temp = Integer.parseInt(sc.next());
			if (max > temp) {
				max = temp;
			}
		}
		int dif = max - min + 1;
		if (dif <= 0) {
			System.out.println("0");
		} else if (dif <= N) {
			System.out.println(dif);
		} else {
			System.out.println(N);
		}

	}

}
/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