#include <stdio.h>

int main() {
	int l, m, a[6000][3], b[6000] = {0}, i, j, count = 0;
	scanf("%d%d", &l, &m);
	for (i = 0; i < m; i++) {
		for (j = 0; j < 2; j++)
			scanf("%d", &a[i][j]);
	}//以上均为定义和输入
	for (j = 0; j < m; j++) {
		for (i = 0; i <= l; i++) {
			if (i >= a[j][0] && i <= a[j][1])
				b[i] = 1;//如果在a[j][0]到a[j][1]的区域把b[i]=1；b[i]的初始值均为0
		}
	}
	for (i = 0; i <= l; i++)
		if (b[i] == 0)
			count++;//统计b[i]=0的个数即为剩余树的数量
	printf("%d", count);
}