import sys
input = sys.stdin.readline

N,L=map(int,input().split())
X=list(map(int,input().split()))
Y=list(map(int,input().split()))

X2=[[x,1] for x in X]
Y2=[[y,2] for y in Y]

ALL=X2+Y2
ALL.sort()

ALL_C=[ALL[0]+[1]]

for i in range(1,2*N):
    if ALL[i][1]==ALL[i-1][1]:
        ALL_C[-1][2]+=1
    else:
        ALL_C.append(ALL[i]+[1])


NOW=0
NOWkind=-1
EAT=0
ANS=0
LEN=len(ALL_C)
i=0

while EAT<2*N:
    #print(ALL_C,NOW,NOWkind,ANS)
    if i==LEN:
        i=0

        for j in range(LEN):
            if ALL_C[j][2]==0:
                continue
            else:
                ALL_C2=[ALL_C[j]]
                break

        for k in range(j+1,LEN):
            if ALL_C[k][2]==0:
                continue
            if ALL_C[k][1]==ALL_C2[-1][1]:
                ALL_C2[-1][2]+=ALL_C[k][2]
            else:
                ALL_C2.append(ALL_C[k])

        if NOWkind==ALL_C2[0][1]==ALL_C2[-1][1]:
            ALL_C2[-1][2]+=ALL_C2[0][2]
            ALL_C2=ALL_C2[1:]

        ALL_C=ALL_C2
        LEN=len(ALL_C)
            

        
    dist,kind,num=ALL_C[i]
    if num==0 or kind==NOWkind:
        i+=1
    else:
        if NOW<dist:
            ANS+=dist-NOW
            ALL_C[i][2]-=1
            EAT+=1
            NOW=dist
            NOWkind=kind
        else:
            ANS+=dist+(L-NOW)
            ALL_C[i][2]-=1
            EAT+=1
            NOW=dist
            NOWkind=kind
        i+=1

print(ANS)