#include<bits/stdc++.h>
using namespace std;int main(){int a[9999],*p=a;cin>>*a;if(*a<=2)cout<<*a;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;}}