我感觉这题有必要发一篇代码（不读题错了 N 次）：

#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=2e5+10;
struct node1{int l,r;bool friend operator<(node1 n1,node1 n2){return n1.l!=n2.l?n1.l>n2.l:n1.r>n2.r;}};
struct node2{int l,r;bool friend operator<(node2 n1,node2 n2){return n1.r!=n2.r?n1.r<n2.r:n1.l<n2.l;}};
multiset<node1>s1;multiset<node2>s2;
node1 a[N];
signed main()
{
	int n;cin>>n;int bk1=0,bk2=0;
	for(int i=1;i<=n;i++)
	{
		int x,y;cin>>x>>y;
		a[i]={x,y};
		if(x>0)bk2=1;if(y<0)bk1=1;
	}
	int ans=0,sum,bk,pos;
	sum=0;bk=0;pos=0;
	s1.clear();s2.clear();
	for(int i=1;i<=n;i++)s1.insert({a[i].l,a[i].r}),s2.insert({a[i].l,a[i].r});
	while(s1.size())
	{
		if(bk)
		{
			auto it=s1.begin();node1 no=*it;
			if(pos<no.l)sum+=no.l-pos,pos=no.l;
			else if(pos>no.r)sum+=pos-no.r,pos=no.r;
			auto it1=s2.lower_bound({no.l,no.r});
			s1.erase(it);s2.erase(it1);
		}
		else
		{
			auto it=s2.begin();node2 no=*it;
			if(pos<no.l)sum+=no.l-pos,pos=no.l;
			else if(pos>no.r)sum+=pos-no.r,pos=no.r;
			auto it1=s1.lower_bound({no.l,no.r});
			s1.erase(it1);s2.erase(it);
		}
		bk^=1;
	}
	if(bk1)ans=max(ans,sum+abs(pos));
	sum=0;bk=1;pos=0;
	s1.clear();s2.clear();
	for(int i=1;i<=n;i++)s1.insert({a[i].l,a[i].r}),s2.insert({a[i].l,a[i].r});
	while(s1.size())
	{
		if(bk)
		{
			auto it=s1.begin();node1 no=*it;
			if(pos<no.l)sum+=no.l-pos,pos=no.l;
			else if(pos>no.r)sum+=pos-no.r,pos=no.r;
			auto it1=s2.lower_bound({no.l,no.r});
			s1.erase(it);s2.erase(it1);
		}
		else
		{
			auto it=s2.begin();node2 no=*it;
			if(pos<no.l)sum+=no.l-pos,pos=no.l;
			else if(pos>no.r)sum+=pos-no.r,pos=no.r;
			auto it1=s1.lower_bound({no.l,no.r});
			s1.erase(it1);s2.erase(it);
		}
		bk^=1;
	}
	if(bk2)ans=max(ans,sum+abs(pos));
	cout<<ans;
	return 0;
}