#include<iostream>
using namespace std;int A[2000];int i;int main(){int N;cin>>N;for(i=0;i<N;i++){cin>>A[i];}int q=1;for(i=0;i<N-1;i++){int d=A[i+1]-A[i];for(int j=i+1;j<N;j++){if(A[j]-A[j-1]!=d){break;}if(q<j-i+1){q=j-i+1;}}}cout<<q<<endl;}