#include<iostream>
#include<cstdio>
#include<vector>
#include <algorithm>

using namespace std;

int main()
{
	int l, m;
	cin >> l >> m;
	vector<int> tree;
	tree.resize(l + 1);
	for (int i = 0; i <= l; i++)
	{
		tree[i] = i;
	}
	for (int i = 0; i < m; i++)
	{
		int u, v;
		cin >> u >> v;
		for (int j = u; j <= v; j++)
		{
			tree.erase(remove(tree.begin(), tree.end(), j), tree.end());
		}
	}
	cout << tree.size() << endl;
	system("pause");
	return 0;
}