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]) # print(xt) 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() tt.pop() for x in g: heappush(gg[0], x+l) # pDB(gg) # pDB(tt) if tt[0] == 1: tt = tt[1:]+tt[:1] gg = gg[1:]+gg[:1] # pDB(gg) # pDB(tt) log = [] ans = 0 now = 0 while gg: ngg = [] ntt = [] last = -1 for t, g in zip(tt, gg): x = -heappop(g) log.append(x) ans += (x-now)%l now = x last = t 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 if tt and tt[0] == tt[-1]: if last == tt[0]: for x in gg[0]: heappush(gg[-1], x-l) gg.pop(0) tt.pop(0) else: for x in gg[-1]: heappush(gg[0], x+l) gg.pop() tt.pop() print(ans) # print(*log, sep="\n") # pre = 0 # s = 0 # for x in log: # if x < 0: x += l # if x-pre > 0: # s += x-pre # else: # s += x-pre+l # pre = x # print(s)