import sys

# sys.setrecursionlimit(1000005)
# sys.set_int_max_str_digits(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-(-1 << 31)
# inf = -1-(-1 << 63)
# md = 10**9+7
md = 998244353

n,m=LI()
aa=LI()
bb=LI()
for i in range(n):
    aa[i]-=bb[i]
    aa[i]%=m
aa.append(0)
# print(aa)
dd=[aa[0]]+[(aa[i+1]-aa[i])%m for i in range(n)]
dd.sort()
l,r=0,n
ans=0
while l<r:
    if dd[l]+dd[r]==m:
        ans+=dd[l]
        l+=1
        r-=1
    elif dd[l]+dd[r]<m:
        ans+=dd[l]
        dd[r]+=dd[l]
        l+=1
    else:
        cur=m-dd[r]
        ans+=cur
        dd[l]-=cur
        r-=1
    # print(l,r,ans,dd)
print(ans)