#include<stdio.h>

int main() {
	int l;
	int m;
	scanf("%d %d", &l, &m);
	if (l>=1&&l<1e4&&m>=1&&m<=100) {
		int head;
		int end;
		int arr[10010];
		for(int i=0;i<=l;i++){
			arr[i] = 0;
		}
		for (int i = 0; i < m; i++) {
			scanf("%d %d", &head, &end);
			
				for (int j = head; j <= end; j++) {
					if (arr[j] == 0) {
						arr[j] = 1;
					}
				}
			
			
		}
		int cnt = 0;
		for (int i = 0; i <=l; i++) {
			if (arr[i] == 0) {
				cnt++;
			}
		}

		printf("%d", cnt);

	}
	return 0;
}