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;
}