n,d=map(int,input().split())
t,k=map(int,input().split())
dpt=t
dpk=k-d
for i in range(n-1):
    t,k=map(int,input().split())
    ndpt=max(dpt+t,dpk-d+t)
    ndpk=max(dpk+k,dpt-d+k)
    dpt=ndpt
    dpk=ndpk
print(max(dpt,dpk))