#include<stdio.h>
int main() {
	int a[1000] = { 0 };
	int l,n;
	scanf("%d %d", &l,&n);
	int j, k,cnt=0;
	for (int x = 1; x <= n; x++) {
		scanf("%d %d", &j, &k);
		for (int p = j; p<= k; p++) {
			a[p] = -1;
		
		}

	}
	for (int i = 0; i <= l; i++) {
		if (a[i] != -1) cnt++;
	}
	printf("%d", cnt);
	return 0;
}