代码{

#include<bits/stdc++.h>
using namespace std;
int l , m , u , v;
bool vis[21331];
int main() {
    cin >> l >> m;
    for (int i = 1; i <= l; i++) vis[i] = false;
    while (m--) {
        cin >> u >> v;
        for (int i = u; i <= v; i++) vis[i] = true;
    }

    int ans = 0;
    for (int i = 1; i <= l; i++) {
        if(vis[i] == false) ans++;
    }
    cout << ans << endl;
    return 0;
}

}