include <bits/stdc++.h>

using namespace std;

int l, n, b, c, d, a[100000];

int main() {

scanf("%d %d", &l, &n);
for (int i = 0; i < n; i++) {
	cin >> b >> c;
	for (int j = b; j <= c; j++) {
		a[j] = 1;
	}
}
for (int i = 0; i <= l; i++) {
	if (!a[i])
		d++;
}
cout << d;

}