#include <iostream>
using namespace std;

int a[10005];

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