import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
md = 10**9+7
# md = 998244353

from heapq import *

n, l = LI()
xx = LI()
yy = LI()
xt = []
for x in xx: xt.append((x, 0))
for x in yy: xt.append((x, 1))
xt.sort(key=lambda x: x[0])

gg = []
tt = []

for x, t in xt:
    if not tt or tt[-1] != t:
        gg.append([])
        tt.append(t)
    heappush(gg[-1], -x)
# pDB(gg)
# pDB(tt)

if tt[0] == tt[-1]:
    g = gg.pop()
    for x in g:
        heappush(gg[0], x+l)
# pDB(gg)
# pDB(tt)

ans = 0
now = 0
while gg:
    ngg = []
    ntt = []
    for t, g in zip(tt,gg):
        x = -heappop(g)
        ans += (x-now)%l
        now = x
        if g:
            if not ntt or ntt[-1] != t:
                ntt.append(t)
                ngg.append(g)
            else:
                for x in g:
                    heappush(ngg[-1], x)
        # pDB(ngg)
        # pDB(ntt)
    tt, gg = ntt, ngg

print(ans)