贪心

按区间左端点从小到大排序，每次维护区间右端点，结果为所维护的区间与当前枚举区间的交集

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

typedef pair<int, int> PII;

int n;
vector<PII> alls;

bool cmp(PII a, PII b)
{
    return a.second < b.second;
}

int main()
{
    cin >> n;
    for (int i = 0; i < 2 * n; i ++ )
    {
        int a, b;
        cin >> a >> b;
        alls.push_back({a, b});
    }

    sort(alls.begin(), alls.end());

    int st = 0, ed = 0, res = 0;
    for (auto seq: alls)
    {
        if (ed < seq.first)
            st = seq.first, ed = seq.second;
        else 
        {
            res += min(ed, seq.second) - seq.first;
            ed = max(ed, seq.second);
        }
    }

    printf("%d\n", res);
    return 0;
}