sub max{$_[$_[1]>$_[0]]}
$N=<>;
@V=glob<>;
@d=(@V[0,1],$V[0]+$V[2]);
$d[$_]=max($d[$_-3],$d[$_-2])+$V[$_]for 3..$N-1;
print$N==1?$d[0]:max(@d[$N-1,$N-2])