#import<bits/stdc++.h>
#define c(a,b)(a=a>b?a:b)
long n,h[3<<17],i,u,v,p[1<<18],q;
main(){
  for(;std::cin>>v;)
    n?h[i++]=(i-n)%2>0?h[u]<h[v]?:u^=v^=u^=v,h[u]*n*n+u*n+v:u=v:n=v+1;
  std::sort(h+1,h-~i);
  p[1]=p[2*n-1]=4*n+1;
  for(i=n+*h;v=h[i++];)
    c(q,c(p[v%n],p[u=v/n%n]+1)+c(p[v%n+n],p[u+n]+1)-8*n);
  printf("%d\n",q-1);
}