/* -*- coding: utf-8 -*-
 *
 * 3543.cc:  No.3543 Progression (All Language) - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 2000;

/* typedef */

/* global variables */

int as[MAX_N], bs[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", as + i);

  if (n <= 2) { printf("%d\n", n); return 0; }

  for (int i = 0; i + 1 < n; i++) bs[i] = as[i + 1] - as[i];

  int m = n - 1;
  int maxl = 0;
  for (int i = 0; i < m;) {
    int j = i;
    while (i < m && bs[j] == bs[i]) i++;
    maxl = max(maxl, i - j + 1);
  }

  printf("%d\n", maxl);
  
  return 0;
}