#include<iostream>

using namespace std;

int s[10010];

int main() {
	
	int n, m;
	cin >> n >> m;
	
	for (int i = 0; i <= n; i ++) s[i] = 1;
	
	while (m -- ) {
		int l, r;
		cin >> l >> r;
		for (int i = l; i <= r; i ++ ) s[i] = 0;
	}
	
	int cnt = 0;
	for (int i = 0; i <= n; i ++ ) {
		if (s[i] != 0) cnt ++ ;
	}
	
	cout << cnt << endl;


	return 0;
}