n,c1 = map(int, input().split())
g = []
c = [0]
d = [0]
a = 0
ans=[]
m1=0
m2=0
for i in range(n):
  x,v = map(int, input().split())
  g.append([x,v])
  a+=v
  m1= max(a-x,m1)
  m2= max(a-2*x,m2)
  c.append(m1)
  d.append(m2)
b = 0
e = [0]
f = [0]
for i,j in reversed(g):
  b+=j
  e.append(b-(c1-i))
  f.append(b-2*(c1-i))
for i in range(n+1):
  ans.append(c[i]+f[-i-1])
  ans.append(d[i]+e[-i-1])
print(max(ans))