include <bits/stdc++.h>

using namespace std;

int main() {

int l, m, c, d;
cin >> l >> m;
int a[l + 1] = {0};
for (int i = 0; i < m; i++) {
	cin >> c >> d;
	for (int j = c; j <= d; j++) {
		a[j] = 1;
	}
}
int sum = 0;
for (int e = 0; e < l + 1; e++) {
	if (a[e] == 0) {
		sum++;
	}
}
cout << sum;
return 0;

}