#include <bits/stdc++.h> using namespace std;

int main() { int a [10010]; int l, n; cin >> l >> n;

for (int i = 0; i <= l; i++)
    a[i] = 1;

while (n--) {
    int s, e;
    cin >> s >> e;

    for (int i = s; i <= e; i++)
        a[i] = 0;
}

int ans = 0;

for (int i = 0; i <= l; i++)
    ans += a[i];

cout << ans << endl;
return 0;

}