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()
    q=[]
    bef=0
    r=0
    for aa,bb in zip(a,b):
        add=(bb-aa-bef)%m
        bef=bb-aa
        r+=add
        heapq.heappush(q,-add)
        
    sum=r
    while r>=m:
        v = -heapq.heappop(q)
        sum+=m-v*2
        r-=m
    ans=sum+r
    v = -heapq.heappop(q)
    sum+=m-v*2
    r=m-r
    ans=min(ans,sum+r)
    
    print(ans//2)
        
if __name__ == "__main__":
    main()