n, d = map(int, input().split())
t, k = map(int, input().split())

dpt = t
dpk = k-d
for i in range(1, n):
    t, k = map(int, input().split())
    ndpt = max(dpt+t, dpk-d+t)
    ndpk = max(dpk+k, dpt-d+k)
    dpt = ndpt
    dpk = ndpk

print(max(dpt, dpk))