#include <bits/stdc++.h>

using namespace std;
//P1047 校门外的树
int main() {
    int L, M;
    cin >> L >> M;

    int array[L];

    for (int i = 0; i <= L; i++) {
        array[i] = 0;
    }

    for (int i = 1; i <= M; i++) {
        int s, t;
        cin >> s >> t;
        for (int j = s; j <= t; j++) {
            array[j] = 1;
        }
    }

    int count = 0;
    for (int i = 0; i <= L; i++) {
        if (array[i] == 0) {
            count++;
        }
    }
    cout << count << endl;
    return 0;
}