#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#define MAX 1005
int main()
{
	int L, M, a[MAX], m, n, count=0,i,j;
	scanf("%d %d", &L, &M);
	for (i = 0; i <= L; i++)
	{
		a[i] = 1;
	}
	for (i = 0; i < M; i++)
	{
		scanf("%d %d", &m, &n);
		for (j = m; j <= n; j++)
		{
			a[j] = 0;
		}
	}
	for (i = 0; i <= L; i++)
	{
		if (a[i] == 1)
		{
			count++;
		}
	}
	printf("%d", count);


	return 0;
}