#include<cstdio>
#include<algorithm>
int n,a[9999],i;main(){scanf("%d",&n);for(i=0;i<n;i++)scanf("%d",&a[i]);a[n]=~0;int ans=1,cnt=2;for(i=0;i<n-1;i++){if(a[i+1]-a[i]==a[i+2]-a[i+1])cnt++;else ans=std::max(ans,cnt),cnt=2;}printf("%d",ans);}