#include <iostream>
using namespace std;

int main() {
	int a[1001];
	int l, m;
	int count = 0;
	cin >> l;
	cin >> m;
	for ( int i = 1; i <= l; i++)
		a[i] = 0;
	for ( int i = 1; i <= m; i++) {
		int c;
		int b;
		cin >> c;
		cin >> b;
		for ( int j = c; j <= b; i++)
			a[i] = 1;
	}
	for ( int i = 1; i <= l; i++) {
		if (a[i] = 0)
			count++;
	}
	cout << count << endl;
	return 0;
}