#import<ios>
int n,i,c,m,a[2000];main(){for(scanf("%d",&n);i<n;scanf("%d",&a[i++]));for(m=n<3?n:i=c=2;i<n;i++)a[i]+a[i-2]-2*a[i-1]?c=2:m=m<++c?c:m;printf("%d",m);}