#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

int main()
{
	int l, n;
	scanf("%d %d", &l, &n);//输入马路的长度和区域的数目
	int s = l + 1;//计算总数目
	int a[10000];
	
	for (int i = 0;i < n;i++)
	{
		int c1 = 0;
		int c2 = 0;
		scanf("%d %d", &c1, &c2);
		for (int i = c1;c1 <= c2;c1++)
		{
			a[c1] = 0;
		}
	}
	
	int res = 0;
	for (int j = 0;j < 10000;j++)
	{
		if (a[j] == 0)
		{
			res++;
		}
	}

	s = s - res;

	printf("%d", s);
	return 0;
}