N, L = list(map(int, input().strip().split()))
X = list(map(int, input().strip().split()))
Y = list(map(int, input().strip().split()))

C = [0 for i in range(L+1)]

for x in X:
    C[x] = 2

for y in Y:
    C[y] = 3

S = [0 for i in range(L+1)]

last = -1
count = 0
for i in range(L+1, 0, -1):
    if C[i-1] != 0:
        count = int(C[i-1] == last) * count + 1
        last = C[i-1]
        S[i-1] = count

ind = 0
m = 0
for i in range(L+1):
    if S[i] >= m and C[i] != 0:
        ind = i
        m = S[i]

print(max(S) * L + ind)