#include<iostream>
#include<cmath>
#include<algorithm>
#include<string>
#include<map>
using namespace std;
int main() {
    int l, m;
    while (cin >> l >> m) {
        int a, b, sum = 0;
        int arr[10000] = { 0 };
        for (int i = 0;i < m;i++) {
            cin >> a >> b;
            for (int j = a;j <= b;j++) {
                arr[j] = 1;
            }
        }
        for (int i = 0;i <= l;i++) {
            if (arr[i] == 0) {
                sum++;
            }
        }
        cout << sum << endl;
    }
    return 0;
}