integer::N,D,i,j=0,place=1,mx,M(4)
integer,allocatable,dimension(:)::T,K
integer,allocatable,dimension(:,:)::sum

read *,N,D
allocate(T(N+1),K(N+1),sum(2,N+1))
do i=2,N+1
 read *,T(i),K(i)
end do

sum(1,1)=0
sum(2,1)=-D

do i=1,N
 sum(1,i+1)=max(sum(1,i),sum(2,i)-D)+T(i+1)
 sum(2,i+1)=max(sum(2,i),sum(1,i)-D)+K(i+1)
end do

print *,max(sum(1,N+1),sum(2,N+1))

end