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

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

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

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

end