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

bool a[10001];

int main()
{
    int l, x, ans = 0;
    cin >> l >> x;
    for (int i = 0; i <= l; ++i)
        a[i] = 1;
    for (int t = 1; t <= x; ++t)
    {
        int n, m;
        cin >> n >> m;
        for (int i = n; i <= m; ++i)
            a[i] = 0;
    }
    for (int i= 0; i <= l; ++i)
        if (a[i]) ans++;
    cout << ans << endl;
    return 0;
}