import sys
import heapq
input = sys.stdin.readline
def read_values(): return tuple(map(int, input().split()))
def read_list(): return list(map(int, input().split()))

def main():  
    n,m=read_values()
    a=read_list()
    b=read_list()
    ba=[0]
    for aa,bb in zip(a,b):        
        ba.append((bb-aa)%m)
    ba.append(0)

    dif=list()
    c=0
    for i in range(n+1):
        dif.append((ba[i+1]-ba[i])%m)
        c+=ba[i+1]<ba[i]
    dif.sort()
    dif.reverse()
    for i in range(c):
        dif[i]=m-dif[i]
    print(sum(dif)//2)
        
        
if __name__ == "__main__":
    main()