80分求助啊，模仿区间合并做的，不知道哪里出现了问题！！

#define  _CRT_SECURE_NO_WARNINGS
#include <iostream>
using namespace std;
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <list>
#include <string>
#include <cmath>
bool mycmp(pair<int, int> a, pair<int, int> b) {
    if (a.first == b.first) {
        return b.second < a.second;
    }
    return a.first < b.first;
}

int main()
{

    int count = 0;
    int l;
    set<pair<int, int>>sort;
    int m;
    cin >> l >> m;
    int x1, x2;
    for (int i = 0; i < m; i++)
    {
        cin >> x1 >> x2;
        sort.insert(make_pair(x1,1));
        sort.insert(make_pair(x2,-1));
    }
    int begin, end;
    int result = 0;
    for (auto e : sort)
    {
        if (count == 0)
            begin = e.first;

        if (e.second == -1)
            end = e.first;

        count +=e.second;
        if (count == 0)
            result += end - begin + 1;

    }

    cout << l+1-result << endl;
    return 0;
}