import sys
input=lambda: sys.stdin.readline().rstrip()
n,d=map(int,input().split())
A=[0,-d]
for _ in range(n):
  t,k=map(int,input().split())
  a1,a2=A
  A=[max(a1+t,a2+t-d),max(a1+k-d,a2+k)]
print(max(A))