#include<cstdio>
#include<iostream>
using namespace std;
int main()
{
	int l, m,u,v;
	int count=0;
	cin >> l >> m;
	int L[10001];
	for (int i = 0; i <= l; i++) L[i] = 0;
	for (int i = 0; i < m; i++)
	{
		cin >> u >> v;
		for (int j = u; j <=v; j++) {
			L[j] = 1;
		}
	}
	for (int i = 0; i <= l; i++)
	{
		if (L[i] == 0) count++;
	}
	cout << count << endl;
	return 0;
}