思路是先正着试一下，再翻转试一下，比较两次大小，以此类推

#include<bits/stdc++.h>
using namespace std;
long long n,a[100001],b[100001],l,r,ans,ls,rs,ll,rr;
int main(){
	//freopen("1.in","r",stdin);
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
		l=b[i];
		r=a[i]-b[i];
		ll=ls;
		rr=rs;
		ls=max(l,ls);
		rs=max(r,rs);
		ans=max(ans,ls+rs);
		ls=max(r,ll);
		rs=max(l,rr);
		ans=max(ans,ls+rs);
		if(ans!=ls+rs){
			ls=max(l,ls);
			rs=max(r,rs);
		}
	}
	cout<<ans;
	return 0;
}