#include<iostream>
using namespace std;
int main(){
	int l,m;
	cin >> l >> m;
	int o = l + 1;
	int a[l + 1] = {};
	int x,y;
	int sum;
	for (int i = 1;i <= m;i++){
		cin >> x >> y;
		for (int j = x;j <= y;j++){
			a[j] = 1;
		}
	}
	for (int i = 1;i <= o;i++){
		if (a[i] == 0){
			sum++;
		}
	}
	cout << sum << endl;
	return 0;
}