#include<bits/stdc++.h>
using namespace std;
int main(){
    int a[9999],*p=a;
    cin>>*a;
    if(*a<=2)cout<<*a<<endl;else{
    while(a[0])cin>>a[a[0]--];
    *p=2;
    while(p[3]){
        p[1]=(p[1]+p[3]==p[2]*2)?*p+1:2;
        *a=max(*a,*++p);
    }
    cout<<*a<<endl;
}}