import sys
input = sys.stdin.readline

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

for i in range(K):
    A.append(A[i]+2*N)

OK=0
NG=N//K+1


while NG>OK+1:
    mid=(OK+NG)//2

    mid2=mid*2

    start=A[1]-mid2+1
    if start<=A[0]:
        start=A[0]+1
    if start%2==1:
        start+=1

    flag=1

    now=start
    for i in range(K):
        now+=mid2
        if now>=A[i+2]:
            flag=0
            break

        if now<=A[i+1]:
            now=A[i+1]+1

        if now%2==1:
            now+=1

    if now-(2*N)>start:
        flag=0

    if flag==1:
        OK=mid
    else:
        NG=mid

print(OK*2)