Progression

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <queue>
#include <map>
#include <set>
#include <cmath>
#include <sstream>
#include <stack>
#include <cassert>

#define pb push_back
#define mp make_pair
#define PI 3.1415926535897932384626433832795
#define sqr(x) (x)*(x)
#define forn(i, n) for(int i = 0; i < n; ++i)
#define ALL(x) x.begin(), x.end()
#define F first
#define S second
#define m0(x) memset(x,0,sizeof(x))
#define CC(x) cout << (x) << "\n"
#define pw(x) (1ull<<(x)).

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int,int> pii;
const int INF = 2147483647;
const ll LLINF = 9223372036854775807LL;

int a[100500];

int main() {
  freopen("progression.in", "r", stdin);
  freopen("progression.out", "w", stdout);
  int n;
  cin >> n;
  forn(i,n) cin >> a[i];
  int l=0;
  int r=1;
  int max=-1;
  while (l<n-1) {
    while (r+1<n && a[r+1]-a[r]==a[l+1]-a[l]) r++;
    if (r-l+1>max) max=r-l+1;
    l=r;
  }
  CC(max);
  return 0;
}