Thailand 4.0

#include<stdio.h>
#include<set>
#include<algorithm>
using namespace std;



typedef long long int ll;
int main()
{
    int n;
    scanf("%d",&n);
    pair<pair<ll,int>,int> Xevn[n*2];
    pair<ll,ll> Yarr[n];
    for(int i = 0 ; i < n ; i ++){
        ll a,b,c,d;
        scanf("%lld %lld %lld %lld",&a,&b,&c,&d);
        Xevn[i]   = {{a,i},+1};
        Xevn[i+n] = {{b,i},-1};
        Yarr[i] = {c,d};
    }
    sort(Xevn , Xevn + n*2);
    multiset<pair<pair<ll,int>,int > > mset;
    ll ans = 0;
    for(int i = 0 ; i < n*2 - 1; i ++){
        int idx = Xevn[i].first.second;
        ll Xa = Xevn[i].first.first;
        ll Xb = Xevn[i+1].first.first;
        int Xstate = Xevn[i].second;
        if(Xstate == 1){
            mset.insert({{Yarr[idx].first ,idx},1});
            mset.insert({{Yarr[idx].second,idx},-1});
        }
        else if(Xstate == -1){
            std::multiset<pair<pair<ll,int>,int > >::iterator its = mset.find({{Yarr[idx].first ,idx},1});
            mset.erase(its);
            its = mset.find({{Yarr[idx].second ,idx},-1});
            mset.erase(its);
        }
        ll cal = 0;
        ll L = 1e9+10,R = -1;
        ll dense = 0;
        std::multiset<pair<pair<ll,int>,int > >::iterator its = mset.begin();
     //   printf("Test i #%d Xl = [%lld,%lld) : \n",i,Xa,Xb);
        while(its != mset.end()){
            ll Yv = its->first.first;
            int Ystate = its->second;
            if(Ystate == 1){
                L = min(L,Yv);
            }else if(Ystate == -1){
                R = max(R,Yv);
            }
            dense += Ystate;
            if(dense == 0){
            //    printf("Y-axis : block at [%lld,%lld)\n",L,R);
                cal += R - L;
                L = 1e9+10;
                R = -1;
            }
            ++its;
        }
      //  printf("Plus : %lld + %lld = %lld\n",ans,(Xb - Xa)*cal,ans+(Xb-Xa)*cal);
        ans += (Xb - Xa) * cal;
    }
    printf("%lld",ans);
    return 0;
}