#include<iostream>
#include<cmath>
#include<algorithm>
using namespace std;
int main()
{
	int l, m;
	cin >> l >> m;
		cout << endl;
		int p[10001],a,b;
		int j = 0;
		for (int i = 0; i <= l; i++)
			p[i] = 1;
		for (int i = 0; i < m; i++)
		{
			cin >> a>> b;
				for (int k = a; k <= b; k++)
			{
				p[k] = 0;
			}
		}
		int count=0;
		for (int t = 0; t <= l; t++)
		{
			if (p[t] !=0)
				count++;
		}
		cout << count;
	return 0;
}