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

p = 't'
m = 0
T = 0
K = -d
for _ in range(n):
    t, k = map(int, input().split())
    T += t
    K += k
    if p == 't' and k - d > t:
        m += k - d
        p = 'k'
    elif p == 'k' and t - d > k:
        m += t - d
        p = 't'
    elif p == 't' and k - d <= t:
        m += t
    elif p == 'k' and t - d <= k:
        m += k

print(max(m, T, K))