// UUID: 15aff100-7e29-4fab-8d5e-aa136850e5c1
#include <bits/stdc++.h>
using namespace std;

int main() {
	int n, m; cin >> n >> m;
	vector<int> le(m);
	vector<int> fel(m);
	for(int i = 0; i < m; i++) {
		cin >> fel[i] >> le[i];
	}
	sort(begin(le), end(le));
	sort(begin(fel), end(fel));
	int packs = 0;
	int ans = 0;
	bool x = false;
	int a = 0;
	int b = 0;
	for(int i = 0; i <m; i++) {
		if(fel[i] == 1) packs++;
		else {
			a = i;
			break;
		}
	}
	for(int i = 0; i <m; i++) {
		if(le[i] == 1) packs--;
		else {
			b = i;
			break;
		}
	}
	for(int i = 2; i<= n; i++) {
		while(a < fel.size()) {
			if(fel[a] == i) {
				a++;
				packs++;
			}
			else {
				break;
			}
		}
		while(b < le.size()) {
			if(le[b] == i) {
				b++;
				packs--;
			}
			else break;
		}
		if(packs == 0 && x == false) {
			ans++;
			x = true;
		}
		else if(packs > 0) x = false;
	}
	cout << ans;
}