#yuki_561

n,d=map(int,raw_input().split())
t,k=map(int,raw_input().split())
dt=t
dk=k-d
for i in xrange(n-1):
 t,k=map(int,raw_input().split())
 tdt=max(dt+t,dk+t-d)
 tdk=max(dk+k,dt+k-d)
 dt=tdt
 dk=tdk
print max(dt,dk)