N, D = map(int, input().split())
t, k = 0, -10**18
for i in range(N):
    T, K = map(int, input().split())
    nt = max(t + T, k + T - D)
    nk = max(k + K, t + K - D)
    t, k = nt, nk
print(max(t, k))