local mma = math.max
local ior = io.read
local n, d = ior("*n", "*n")
local p1, p2 = {}, {}
local t, k = ior("*n", "*n")
p1[1], p2[1] = t, k - d
for i = 2, n do
  t, k = ior("*n", "*n")
  p1[i] = mma(p1[i - 1] + t, p2[i - 1] + t - d)
  p2[i] = mma(p1[i - 1] + k - d, p2[i - 1] + k)
end
print(mma(p1[n], p2[n]))