#include<iostream>
using namespace std;

int main()
{
	int l, m, u, v, tree = 0;
	cin >> l >> m;
	int trees[10000] = {};
	for (int i = 0; i <= l; i++)
		trees[i] = 1;
	for (int i = 1; i <= m; i++)
	{
		cin >> u >> v;
		for (; u <= v; u++)
			trees[u] = 0;
	}
	for (int i = 0; i < 10000; i++)
		tree += trees[i];
	cout << tree << endl;
}