#include<cstdio>
int x,a[9999],v=1,i=1;main(){for(scanf("%d%d",&x,&a[0]);~scanf("%d",&a[i]);i++)x=i>=2&&a[i]+a[i-2]==a[i-1]*2?x+1:2,v=v<x?x:v;printf("%d",v);}