sub max{$_[0]>$_[1]?$_[0]:$_[1]}
($n,$d)=glob<>;
$t=0;$k=-$d;
for(<>){
	/ /;
	($t,$k)=(max($t+$`,$k+$'-$d),max($t+$`-$d,$k+$'));
}
print max($t,$k);