#include <iostream>
#include<algorithm>
using namespace std;
int main(void)
{
	int l, m = 0;
	cin >> l >> m;
	int u, v = 0;
	int num , res = 0;
	int left = 0;
	int right = 0;
	int others = 0;
	for (int i = 1; i <= m; i++)
	{
		cin >> u >> v;
		if (i == 1)
		{
			left = u;
			right = v;
			continue;
		}
		if ((v < left) || (u > right))
		{
			others += v - u + 1;
		}
		else
		{
			left = min(u, left);
			right = max(v, right);
		}
	}
	num = right-left + 1;
	res = l - num - others + 1;
	cout << res << endl;

	return 0;
}