#import<bits/stdc++.h>
#define c(a,b)(a=a>b?a:b)
long n,h[4<<17],i,u,v,q,j;main(){for(;std::cin>>v;)n?h[i++]=(i-n)%2>0?h[u]<h[v]?:u^=v^=u^=v,h[v]*n*n+u*n+v:u=v:n=v+1;j=*h+n;std::sort(h,h+i);for(h[1]=h[n+i++]=4*n+1;v=h[j++];)c(q,c(h[v%n],h[u=v/n%n]+1)+c(h[v%n+i],h[u+i]+1)-8*n);printf("%d\n",q-1);}