#include<cstdio>
#include<algorithm>
#include<vector>
using namespace std;
const int N=5005;
const int Z=2;
const int INF=1e9;
int n,ll[N],rr[N];
int fa[N];
vector<int>son[N];
inline int Read(){
	char ch;
	int f=1;
	while((ch=getchar())<'0'||ch>'9')
		if(ch=='-') f=-1;
	int x=ch^48;
	while((ch=getchar())>='0'&&ch<='9')
		x=(x<<3)+(x<<1)+(ch^48);
	return x*f;
}
inline void print(int x){
	if(x>=10) print(x/10);
	putchar(x%10+48);
	return ;
}
inline void Print(int x,char ch='\n'){
	if(x<0){
		putchar('-');
		print(-x);
	}
	else print(x);
	putchar(ch);
	return ;
}
inline bool Cmp(int x,int y){
	return ll[x]<ll[y];
}
inline void Init(){
	n=Read();
	for(int i=1;i<=n;i++){
		ll[i]=Read();
		rr[i]=Read();
	}
	ll[0]=-1,rr[0]=INF+1;
	for(int i=1;i<=n;i++){
		fa[i]=0;
		for(int j=1;j<=n;j++){
			if(ll[j]>=ll[i]) continue ;
			if(rr[j]<=rr[i]) continue ;
			if(rr[j]-ll[j]<rr[fa[i]]-ll[fa[i]]) fa[i]=j;
		}
		if(fa[i]) son[fa[i]].push_back(i);
	}
	for(int i=1;i<=n;i++)
		sort(son[i].begin(),son[i].end(),Cmp);
	return ;
}
inline int Max(int x,int y){
	return x>y?x:y;
}
inline int Min(int x,int y){
	return x<y?x:y;
}
inline void Cmax(int&x,int y){
	if(y>x) x=y;
	return ;
}
inline void Cmin(int&x,int y){
	if(y<x) x=y;
	return ;
}
int f[N][N][Z][Z];
int mx[N];
inline void Dfs(int u){
	if(!son[u].size()){
		mx[u]=0;
		for(int i=0;i<=1;i++)
			for(int j=0;j<=1;j++)
				f[u][0][i][j]=-INF;
		f[u][0][0][0]=rr[u]-ll[u];
		return ;
	}
	for(int i=0;i<son[u].size();i++){
		int v=son[u][i];
		Dfs(v);
	}
	mx[u]=1;
	for(int i=0;i<2;i++)
		for(int j=0;j<2;j++)
			f[u][0][i][j]=f[u][1][i][j]=-INF;
	f[u][0][0][0]=0;
	f[u][1][1][1]=ll[son[u].front()]-ll[u];
	for(int ii=0;ii<son[u].size();ii++){
		int v=son[u][ii];
		for(int i=0;i<=mx[u]+mx[v]+1&&i<=n;i++)
			for(int ul=0;ul<=1;ul++)
				for(int ur=0;ur<=1;ur++)
					f[0][i][ul][ur]=-INF;
		for(int i=0;i<=mx[u];i++)
			for(int ul=0;ul<=1;ul++)
				for(int ur=0;ur<=1;ur++){
					if(f[u][i][ul][ur]<0) continue ;
					for(int j=0;j<=mx[v]&&i+j-1<=n;j++)
						for(int vl=0;vl<=1;vl++)
							for(int vr=0;vr<=1;vr++){
								if(f[v][j][vl][vr]<0) continue ;
								Cmax(f[0][i+j-(ur&&vl)][ul][0],f[u][i][ul][ur]+f[v][j][vl][vr]);
								int len;
								if(ii==son[u].size()-1) len=rr[u]-rr[v];
								else len=ll[son[u][ii+1]]-rr[v];
								Cmax(f[0][i+j-(ur&&vl)+(!vr)][ul][1],f[u][i][ul][ur]+f[v][j][vl][vr]+len);
							}
				}
		mx[u]+=mx[v]+1;
		if(mx[u]>n) mx[u]=n;
		for(int i=0;i<=mx[u];i++)
			for(int ul=0;ul<=1;ul++)
				for(int ur=0;ur<=1;ur++)
					f[u][i][ul][ur]=f[0][i][ul][ur];
	}
	/*for(int i=0;i<=mx[u];i++)
		for(int ul=0;ul<=1;ul++)
			for(int ur=0;ur<=1;ur++)
				if(f[u][i][ul][ur]>=0)
					printf("before u=%d i=%d ul=%d ur=%d %d\n",u,i,ul,ur,f[u][i][ul][ur]);*/
	for(int i=0;i<mx[u];i++){
		for(int ul=0;ul<=1;ul++)
			for(int ur=0;ur<=1;ur++)
				f[u][i][ul][ur]=-INF;
		f[u][i][0][0]=Max(f[u][i+1][0][0],Max(f[u][i+1][0][1],f[u][i+1][1][0]));
		if(i>=1) f[u][i][0][1]=Max(f[u][i+1][0][1],f[u][i+1][1][1]);
		if(i>=1) f[u][i][1][0]=Max(f[u][i+1][1][0],f[u][i+1][1][1]);
		if(i>=2) f[u][i][1][1]=f[u][i+1][1][1];
	}
	mx[u]--;
	return ;
}
int ans;
inline bool Solve1(){
	for(int i=1;i<n;i++)
		if(ll[i]>=rr[i+1]||rr[i]<=rr[i+1]) return 0;
	ans=rr[1]-ll[1]-Min(ll[2]-ll[1],rr[1]-rr[2]);
	Print(ans);
	return 1;
}
inline void Solve(){
	if(n>400&&Solve1()) return ;
	for(int i=1;i<=n;i++)
		if(!fa[i]){
			Dfs(i);
			ans+=f[i][0][0][0];
		}
	/*for(int u=1;u<=n;u++)
		for(int i=0;i<=mx[u];i++)
			for(int ul=0;ul<=1;ul++)
				for(int ur=0;ur<=1;ur++)
					if(f[u][i][ul][ur]>=0)
						printf("u=%d i=%d ul=%d ur=%d %d\n",u,i,ul,ur,f[u][i][ul][ur]);*/
	return Print(ans);
}
#include<ctime>
int main(){
	//freopen("segment.in","r",stdin);
	//freopen("segment.out","w",stdout);
	//#define LOCAL
	#ifdef LOCAL
	int st=clock();
	#endif
	Init();
	Solve();
	#ifdef LOCAL
	int en=clock();
	printf("cost %d ms\n",en-st);
	#endif
	return 0;
}