我的代码：

#include <iostream>
#include <cstring>
using namespace std;

bool t[10000];

int main()
{
    int l, m, cnt = 0;
    cin >> l >> m;
    memset(t, 1, sizeof(t));
    for (int i = 1; i <= m; ++i)
    {
        int a, b;
        cin >> a >> b;
        for (int i = a; i <= b; ++i)
        {
            t[i] = 0;
        }
    }
    for (int i = 0; i <= l; ++i)
    {
        if (t[i]) cnt++;
    }
    cnt++;
    cout << cnt;
}