#include <bits/stdc++.h>
using namespace std;
int n , m , l , r;
bool vis[10110];
int main() {
	cin >> n >> m;
	for(int i = 1;i <= n;++i) {
		vis[i] = false;
	}
	while(m--) {
		cin >> l >> r;
		for(int i = l;i <= r;++i) {
			vis[i] = true;
		}
	}
	int ans = 0;
	for(int i = 1;i <= n;++i) {
		if(vis[i] == false) ans++;
	}
	cout << ans << endl;
	return 0;
}