#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n,a,k1,k2,s[10001] = {},g = 0;
	cin >> n >> a;
	for (int i = 1;i <= a; i++) {
		cin >> k1 >> k2;
		for (int j = k1;j <= k2; j++) {
			s[j] = 1;
		}
	}
	for (int i = 0;i <= n; i++) {
		if (s[i] == 0) {
			g ++;
		}
	}
	cout << g;
	return 0;
}