#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstdlib>

#include <map>
#include <set>
#include <queue>
#include <string>
#include <iostream>
#include <algorithm>

using namespace std;

#define maxn 50005
#define inf 0x3f3f3f3f
#define F(i,j,k) for (int i=j;i<=k;++i)
#define D(i,j,k) for (int i=j;i>=k;--i)

void Finout()
{
    #ifndef ONLINE_JUDGE
    freopen("in.txt","r",stdin);
//    freopen("out.txt","w",stdout);
    #endif
}

int Getint()
{
    int x=0,f=1; char ch=getchar();
    while (ch<‘0‘||ch>‘9‘) {if (ch==‘-‘) f=-1; ch=getchar();}
    while (ch>=‘0‘&&ch<=‘9‘) {x=x*10+ch-‘0‘; ch=getchar();}
    return x*f;
}

struct SuffixArray{
	int s[maxn];
	int tmp[maxn],cnt[maxn],sa[maxn],rk[maxn],h[maxn];
	void build(int n,int m)
	{
		int i,j,k;n++;
		F(i,0,2*n+5) tmp[i]=sa[i]=rk[i]=h[i]=0;
		F(i,0,m-1) cnt[i]=0;
		F(i,0,n-1) cnt[rk[i]=s[i]]++;
		F(i,1,m-1) cnt[i]+=cnt[i-1];
		F(i,0,n-1) sa[--cnt[rk[i]]]=i;
		for (k=1;k<=n;k<<=1)
		{
			F(i,0,n-1)
			{
				j=sa[i]-k;
				if (j<0) j+=n;
				tmp[cnt[rk[j]]++]=j;
			}
			sa[tmp[cnt[0]=0]]=j=0;
			F(i,1,n-1)
			{
				if (rk[tmp[i]]!=rk[tmp[i-1]]||rk[tmp[i]+k]!=rk[tmp[i-1]+k]) cnt[++j]=i;
				sa[tmp[i]]=j;
			}
			memcpy(rk,sa,n*sizeof(int));
			memcpy(sa,tmp,n*sizeof(int));
			if (j>=n-1) break;
		}
		for (j=rk[h[i=k=0]=0];i<n-1;++i,++k)
			while (~k&&s[i]!=s[sa[j-1]+k]) h[j]=k--,j=rk[sa[j]+1];
	}
}arr;

int N,a[maxn];

bool test(int k,int n)
{
	int minn=arr.sa[1],maxx=arr.sa[1];
	F(i,2,n)
	{
		if (arr.h[i]>=k&&i<n)
		{
			minn=min(arr.sa[i],minn);
			maxx=max(arr.sa[i],maxx);
			continue;
		}
		if (maxx-minn>=k) return true;
		minn=arr.sa[i];
		maxx=arr.sa[i];
	}
	return false;
}

int main()
{
    Finout();
    while (scanf("%d",&N)!=EOF&&N)
    {
    	F(i,0,N-1) scanf("%d",&a[i]);
    	F(i,0,N-2) arr.s[i]=a[i]-a[i+1]+89;
    	arr.s[N-1]=0;
    	arr.build(N-1,200);
    	int l=3,r=(N-2)/2;
    	while (l<r)
    	{
    		int mid=(l+r)/2+1;
    		if (test(mid,N-1)) l=mid;
    		else r=mid-1;
		}
		printf("%d\n",l<4?0:l+1);
	}
}