#include <bits/stdc++.h>
using namespace std;

int main(){
	int l, m, f[10010], ans = 0;
	cin >> l >> m;
	for (int i = 0; i <= l; i++) {
		f[i] = 1;
	}
	for (int i = 1; i <= m; i++) {
		int u, v;
		cin >> u >> v;
		for (int k = u; k <= v; k++) {
			f[k] = 0;
		}
	}
	for (int i = 0; i <= l; i++) {
		if (f[i] == 1) {
			ans++;
		}
	}
	cout << ans;
	return 0;
}