n,d=map(int,input().split())
t,k=map(int,input().split())
ft=t
fk=k-d
for i in range(n-1):
    t,k=map(int,input().split())
    t_i=max(ft,fk-d)+t
    k_i=max(ft-d,fk)+k
    ft=t_i
    fk=k_i
print(max(ft,fk))