啥也不去想，写就完事了。。

#include<iostream>
#include<algorithm>
using namespace std;
const int N = 1e4 + 5;
int arr[N];
int main() {
    int L,M;
    cin >> L >> M;
    int ans = 0,x,y;
    for (int i = 1;i <= M;i++) {
        cin >> x >> y;
        for (int j = x;j <= y;j++) {
            arr[j] = 1;
        }
    }
    for (int i = 0;i <= L;i++) {
        if (!arr[i]) ans++;
    }
    cout << ans << endl;
    return 0;
}