#include<iostream>
using namespace std;
int a[1005];
int m, l;
int main()
{
    cin >> l >> m;
    for (int i = 1; i <= m; i++)
    {
        int x, y;
        cin >> x >> y;
        for (int j = x; j <= y; j++)a[j] = 1;
    }
    int cnt = 0;
    for (int i = 1; i <= l; i++)
        if (a[i] == 0)
            cnt++;
    cout << cnt << endl;
    return 0;
}