#include<iostream>

using namespace std;

int tree[1005],m,l,a,b,cnt;
int main(){
	cin >> l >> m;
	for(int i = 1; i <= m; i++){
		cin >> a >> b;
		for(int j = a; j <= b; j++){
			tree[j] = 1;
		}
	}
	for(int i = 0; i <= l; i++){
		if(!tree[i]) cnt++;
	}
	cout << cnt << endl;
	return 0;
}