#include<iostream>
using namespace std;
int main()
{
	int l = 0, m = 0, a = 0, b = 0, trees = 0, i = 0;
	int arr[10000]{};
	cin >> l >> m;
	for (i = 0; i <= l; i++)
	{
		arr[i] = 0;
	}
	while(m--)
	{
		cin >> a >> b;
		for (i = a; a <= b; a++)
		{
			arr[i] = -1;
		}
	}
	for (i = 0; i <= l; i++)
	{
		if (arr[i] == 0)
			trees++;
	}
	cout << trees;

	return 0;
}