#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
#include<bits/stdc++.h>
using namespace std;
int l,a[10000],m,n=0;
int main()
{
	scanf("%d %d", &l,&m);
	memset(a, 0, sizeof(a));
	for (int i = 1; i <= m; i++)
	{
		int u, v;
		scanf("%d %d", &u, &v);
		for (int j = u; j <= v; j++)a[j] = 1;
	}
	for (int i = 0; i <= l; i++)
		if (a[i] == 0)n++;
	printf("%d\n", n);
	return 0;
}