#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<ctime>
#include<utility>
#include<algorithm>
#include<vector>
#include<queue>
#include<set>
#include<map>
#include<cmath>
#include<cstring>
#include<cctype>
#include<string>
#include<string.h>
#include<list>
#include<strstream>
using namespace std;

struct node
{
	int now,next;
};

int n;
int x[20005];
int rank[20005],sa[20005];
int pos[20005],val[2][20005],c[20005];
node d[20005];
int height[20005],h[20005];

int min(int x,int y)
{
	return x<y ? x : y;
}

int max(int x,int y)
{
	return x>y ? x : y;
}

void add_value(int u,int v,int i)
{
	d[i].next=c[u];c[u]=i;
	d[i].now=v;
}

void radix_sort(int l,int r)
{
	for(int k=1;k>=0;k--)
	{
		memset(c,0,sizeof(c));
		for(int i=r;i>=l;i--)
		{
			add_value(val[k][pos[i]],pos[i],i);
		}
		
		int t=0;
		for(int i=0;i<=20000;i++)
		{
			for(int j=c[i];j;j=d[j].next)
				pos[++t]=d[j].now;
		}
	}
	
	int t=0;
	for(int i=1;i<=n;i++)
	{
		if(val[0][pos[i]]!=val[0][pos[i-1]] || val[1][pos[i]]!=val[1][pos[i-1]]) t++;
		rank[pos[i]]=t;
	}
}

void get_suffix_array()
{
	int t=1;
	while(t/2<=n) //******
	{
		for(int i=1;i<=n;i++)
		{
			val[0][i]=rank[i];
			if(i+t/2<=n) val[1][i]=rank[i+t/2]; else val[1][i]=0;
			pos[i]=i;
		}
		
		radix_sort(1,n);
		t*=2;
	}
	
	for(int i=1;i<=n;i++) sa[rank[i]]=i;
}

//-------------------------------------------------------------------------------

void get_common_prefix()
{
	memset(h,0,sizeof(h));
	for(int i=1;i<=n;i++)
	{
		if(rank[i]==1) h[i]=0;
		else
		{
			int now=0;
			if(i>1 && h[i-1]>1) now=h[i-1]-1;
			while(now+i<=n && now+sa[rank[i]-1]<=n && x[now+i]==x[now+sa[rank[i]-1]]) now++;
			h[i]=now;
		}
	}
	for(int i=1;i<=n;i++) height[rank[i]]=h[i];
}

//-------------------------------------------------------------------------------

bool exist(int len)
{
	int minn=n+1,maxx=0;
	for(int i=1;i<=n;i++)
	{
		if(height[i]<len)
		{
			if(maxx-minn>=len) return true;
			minn=maxx=sa[i];
		}
		else
		{
			minn=min(minn,sa[i]);
			maxx=max(maxx,sa[i]);
		}
	}
	if(maxx-minn>=len) return true;
	return false;
}

int binary_search(int ln,int rn)
{
	while(rn-ln>1)
	{
		int mid=(ln+rn)>>1;
		if(exist(mid)) ln=mid; else rn=mid;
	}
	return ln;
}

int main()
{
	scanf("%d",&n);
	while(n!=0)
	{
		for(int i=1;i<=n;i++) scanf("%d",&x[i]);
		
		for(int i=1;i<=n-1;i++)
		x[i]=rank[i]=x[i+1]-x[i]+88;
		
		n--;
		get_suffix_array();
		get_common_prefix();
		int ans=binary_search(0,n)+1;
		printf("%d\n",ans<5 ? 0 : ans);
		
		scanf("%d",&n);
	}
	return 0;
}