#include <iostream>
#include <cstdio>
#include <sstream>
#include <cstring>
#include <map>
#include <cctype>
#include <set>
#include <vector>
#include <stack>
#include <queue>
#include <algorithm>
#include <cmath>
#define rap(i, a, n) for(int i=a; i<=n; i++)
#define rep(i, a, n) for(int i=a; i<n; i++)
#define lap(i, a, n) for(int i=n; i>=a; i--)
#define lep(i, a, n) for(int i=n; i>a; i--)
#define rd(a) scanf("%d", &a)
#define rlld(a) scanf("%lld", &a)
#define rc(a) scanf("%c", &a)
#define rs(a) scanf("%s", a)
#define MOD 2018
#define LL long long
#define ULL unsigned long long
#define Pair pair<int, int>
#define mem(a, b) memset(a, b, sizeof(a))
#define _  ios_base::sync_with_stdio(0),cin.tie(0)
//freopen("1.txt", "r", stdin);
using namespace std;
const int maxn = 100010, INF = 0x7fffffff;int a[maxn], s[maxn];
int sa[maxn], t[maxn], t2[maxn], c[maxn], n;
int ran[maxn], height[maxn];void get_sa(int m)
{int i, *x = t, *y = t2;for(i = 0; i < m; i++) c[i] = 0;for(i = 0; i < n; i++) c[x[i] = s[i]]++;for(i = 1; i < m; i++) c[i] += c[i-1];for(i = n-1; i >= 0; i--) sa[--c[x[i]]] = i;for(int k = 1; k <= n; k <<= 1){int p = 0;for(i = n-k; i < n; i++) y[p++] = i;for(i = 0; i < n; i++) if(sa[i] >= k) y[p++] = sa[i] - k;for(i = 0; i < m; i++) c[i] = 0;for(i = 0; i < n; i++) c[x[y[i]]]++;for(i = 0; i< m; i++) c[i] += c[i-1];for(i = n-1; i >= 0; i--) sa[--c[x[y[i]]]] = y[i];swap(x, y);p = 1; x[sa[0]] = 0;for(i = 1; i < n; i++)x[sa[i]] = y[sa[i-1]] == y[sa[i]] && y[sa[i-1]+k] == y[sa[i]+k] ? p-1 : p++;if(p >= n) break;m = p;}int k = 0;for(i = 0; i < n; i++) ran[sa[i]] = i;for(i = 0; i < n; i++){if(k) k--;int j = sa[ran[i]-1];while(s[i+k] == s[j+k]) k++;height[ran[i]] = k;}
}bool solve(int k)
{int minn = INF, maxx = -INF;rep(i, 1, n){if(height[i] >= k){minn = min(minn, min(sa[i], sa[i-1]));maxx = max(maxx, max(sa[i], sa[i-1]));if(maxx - minn >= k) return true;}elseminn = INF, maxx = -INF;}return false;
}int main()
{while(~rd(n) && n){rep(i, 0, n)rd(a[i]);if(n == 1){puts("0");continue;}rep(i, 0, n-1)s[i] = a[i+1] - a[i] + 100;s[n-1] = 0;get_sa(200);int l = 0, r = n;while(l <= r){int mid = l + (r - l) / 2;if(solve(mid)) l = mid + 1;else r = mid - 1;}if(l >= 4) cout<< r+1 <<endl;else cout<< "0" <<endl;}return 0;
}