考验（NDK1330）

program NDK1330;



const

 maxn=4000;



var

 n,max:longint;

 a,f:array [0..maxn] of longint;

 

procedure bbaa(x:longint);

var

 i,j,k:longint;

begin

 for i:=x-1 downto max+2 do

  begin

   if a[i]=a[x] then

    begin

     for j:=i-1 downto max+1 do

      begin

       for k:=j-1 downto max do 

        begin

         if a[k]=a[j] then

          begin

           f[x]:=f[max]+1;

           max:=x+1;

           exit;           

          end;

        end;

      end;

    end;

  end;

end;



procedure baba(x:longint);

var

 i,j,k:longint;

begin

 for i:=x-1 downto max+2 do

  begin

   for j:=i-1 downto max+1 do

    begin

     if a[j]=a[x] then

      begin

       for k:=j-1 downto max do

        begin

         if a[k]=a[i] then

          begin

           f[x]:=f[max]+1;

           max:=x+1;

           exit;           

          end;

        end;

      end;

    end;

  end;

end;



procedure abba(x:longint);

var

 i,j,k:longint;

begin

 for i:=x-1 downto max+2 do

  begin

   for j:=i-1 downto max+1 do

    begin

     if a[j]=a[i] then

      begin

       for k:=j-1 downto max do

        begin

         if a[k]=a[x] then

          begin

           f[x]:=f[max]+1;

           max:=x+1;

           exit;

          end;

        end;

      end;

    end;

  end;

end;

 

procedure init;

var

 i:longint;

begin

 readln(n);

 for i:=1 to n do read(a[i]);

 max:=1;

 fillchar(f,sizeof(f),0);

 for i:=4 to n do

  begin

   bbaa(i);

   if f[i]>0 then continue;

   baba(i);

   if f[i]>0 then continue;

   abba(i);

   if f[i]>0 then continue;

   f[i]:=f[i-1];

  end;

end;



begin

 assign(input,'NDK1330.in'); reset(input);

 assign(output,'NDK1330.out'); rewrite(output);

 

 init;

 writeln(f[n]);

 

 close(input); close(output);

end.