#include <bits/stdc++.h>
using namespace std;

const int N = 10010;

int L, m;
bool st[N];

int main() {
    cin >> L >> m;
    while (m--) {
        int l, r;
        cin >> l >> r;
        for (int i = l; i <= r; i++) st[i] = 1;
    }
    int res = 0;
    for (int i = 0; i <= L; i++)
        if (!st[i]) res++;
    printf("%d\n", res);
    return 0;
}