#include <iostream>
#include<algorithm>
using namespace std;
int main(void)
{
    int l, m = 0;
    cin >> l >> m;
    int u, v = 0;
    int num , res = 0;
    int left = 0;
    int right = 0;
    int others = 0;
    for (int i = 1; i <= m; i++)
    {
        cin >> u >> v;
        if (i == 1)
        {
            left = u;
            right = v;
            continue;
        }
        if ((v < left) || (u > right))
        {
            others += v - u + 1;
        }
        else
        {
            left = min(u, left);
            right = max(v, right);
        }
    }
    num = right-left + 1;
    res = l - num - others + 1;
    cout << res << endl;

    return 0;
}