N, D = map(int, input().split())

pos = 0
sum = 0
for i in range(len(N)):
    t, k = map(int, input().split())
    if t > k:
        if pos == 0:
            sum += t
        else:
            if (k - t) > D:
                pos = 1
                sum += (k - D)
            else:
                sum += t
    else:
        if pos == 1:
            sum += k
        else:
            if (t - k) > D:
                pos = 0
                sum += (t - D)
            else:
                sum += k

print(sum)