n, m = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))
a.append(0)
b.append(0)
sm = 0
costs = []
pre = 0
for x, y in zip(a, b):
    z = (y - x) % m
    v = (z - pre) % m
    sm += v
    costs.append(m - v)
    pre = z
cnt = sm // m
costs.sort()
ans = sum(costs[:cnt])
print(ans)