#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;
}