def merge(x, y):
    x.reverse()
    y.reverse()
    res = []
    while x and y:
        if x[-1] < y[-1]:
            res.append(x.pop())
        else:
            res.append(~y.pop())
    while x:
        res.append(x.pop())
    while y:
        res.append(~y.pop())
    return res


N, L = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))
C = merge(A[:], B[:])

p = 0
D = [-1] * (2 * N)
for i, c in enumerate(C):
    if c >= 0:
        p += 1
    else:
        p -= 1
    D[i] = p

mx = max(D)
mn = min(D)

if mx == 0:
    ans = L * (mx - mn) + B[0]
else:
    D = [d - mn for d in D]
    loop = max(D) - 1
    walk = -1
    for c, d in zip(C, D):
        if c < 0 and d == loop:
            walk = ~c
    ans = L*loop + walk

print(ans)