use strict;
use warnings;

my $num = <STDIN>;
my %sushi;
for(reverse split(' ', <STDIN>)){
    $sushi{$num} = $_;
    $num--;
}

my %p = (-1 => 0);
my %n = %p;

for my $i (sort {$a <=> $b} keys %sushi) {
  for my $j (keys %p) {
    if($i - $j != 1){
      if(!defined($n{$i}) or (defined($n{$i}) and $n{$i} < $p{$j} + $sushi{$i})){
        $n{$i} = $p{$j} + $sushi{$i};
      }
    }
  }
  %p = %n;
}

print $p{(sort {$p{$b} <=> $p{$a}} keys %p)[0]};