#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<string>
#include<cstring>
#include<cmath>
#include<ctime>
#include<algorithm>
#include<iomanip>
#include<set>
#include<map>
#include<queue>
using namespace std;
#define mem1(i,j) memset(i,j,sizeof(i))
#define mem2(i,j) memcpy(i,j,sizeof(i))
#define LL long long
#define up(i,j,n) for(int i=(j);i<=(n);i++)
#define FILE "dealing"
#define poi vec
#define eps 1e-10
#define db double 
const int maxn=40005,inf=1000000000,mod=1000000007;
int read(){
	int x=0,f=1,ch=getchar();
	while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
	while(ch<=‘9‘&&ch>=‘0‘){x=(x<<1)+(x<<3)+ch-‘0‘,ch=getchar();}
	return f*x;
}
bool cmax(int& a,int b){return a<b?a=b,true:false;}
bool cmin(int& a,int b){return a>b?a=b,true:false;}
int a[maxn],l[maxn],r[maxn];

int cnt=1,now=1;
int pre[maxn],len[maxn],ct[maxn],sa[maxn],np,p,q,nq;
int c[maxn][180];
void extend(int x){
	p=now;np=++cnt;len[np]=len[p]+1;now=np;l[np]=r[np]=len[np];
	while(p&&!c[p][x])c[p][x]=np,p=pre[p];
	if(!p)pre[np]=1;
	else {
		q=c[p][x];
		if(len[q]==len[p]+1)pre[np]=q;
		else {
			len[nq=++cnt]=len[p]+1;
			mem2(c[nq],c[q]);
			pre[nq]=pre[q];
			pre[q]=pre[np]=nq;
			while(p&&c[p][x]==q)
				c[p][x]=nq,p=pre[p];
		}
	}
}
void getsort(){
	mem1(ct,0);mem1(sa,0);
	up(i,1,cnt)ct[len[i]]++;
	up(i,1,cnt)ct[i]+=ct[i-1];
	for(int i=cnt;i>=1;i--)sa[ct[len[i]]--]=i;
}
void clear(){
	mem1(pre,0);mem1(len,0);cnt=1,now=1;mem1(c,0);
}
void build(int* s,int n){
	clear();
	up(i,1,n)
		extend(s[i]);
}

int main(){
	freopen(FILE".in","r",stdin);
	freopen(FILE".out","w",stdout);
	while(true){
		mem1(l,10);mem1(r,0);
		int n=read();if(!n)break;
		up(i,1,n)a[i]=read();
		up(i,1,n-1)a[i]=a[i+1]-a[i]+87;n--;
		build(a,n);
		getsort();int *t=sa;
		for(int i=cnt;i>=1;i--){
			r[pre[t[i]]]=max(r[pre[t[i]]],r[t[i]]);
			l[pre[t[i]]]=min(l[pre[t[i]]],l[t[i]]);
		}
		int ans=-1e9;
		up(i,2,cnt)ans=max(ans,min(r[i]-l[i],len[i]));
		printf("%d\n",ans<4?0:ans+1);
	}
	return 0;
}