N, D = map(int, input().split())
T, K = map(int, input().split())
dpT = T
dpK = K - D
for n in range(N - 1):
    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))