N,mod=map(int,input().split())
A=list(map(int,input().split()))
B=list(map(int,input().split()))

for i in range(N):
    A[i]=(A[i]-B[i])%mod

A=[0]+A+[0]

S=[]
for i in range(1,len(A)):
    S.append((A[i]-A[i-1]))

SUM=0
for s in S:
    if s>=0:
        SUM+=s

S.sort(reverse=True)

for i in range(len(S)):
    if S[i]>S[-1-i]+mod:
        SUM=SUM-S[i]+S[-1-i]+mod

print(SUM)