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

tokyo, kyoto = t, k - d

for i in range(n-1):
    t, k = map(int, input().split())
    t_i = max(tokyo, kyoto - d) + t
    k_i = max(tokyo - d, kyoto) + k
    tokyo = t_i
    kyoto = k_i
    
print(max(tokyo, kyoto))