import sys

# sys.setrecursionlimit(200005)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

md = 10**9+7
# md = 998244353

n,k=LI()
aa=LI()

n2=n*2
aa+=[a+n2 for a in aa[:2]]
# si=max(range(k),key=lambda i:(aa[i]-aa[i-1])%n2)
# aa=aa[si:si+k+1]
# print(aa)

def binary_search(l, r, minimize):
    if minimize: l -= 1
    else: r += 1
    while l+1 < r:
        m = (l+r)//2
        if ok(m) ^ minimize: l = m
        else: r = m
    if minimize: return r
    return l

def ok(m):
    m*=2
    to=[inf]*n2*2
    for j in range(aa[k-1]+1,aa[k],2):
        nj=max(j+m,aa[k]+1)
        if nj>=aa[k+1]:continue
        to[j]=nj
    for i in range(k-1)[::-1]:
        l,r,r2=aa[i],aa[i+1],aa[i+2]
        for j in range(l+1,r,2):
            nj=max(r+1,j+m)
            if nj>=r2:continue
            to[j]=to[nj]
            if i==0 and to[j]<=j+n2:return True
    return False

ans=binary_search(0,n//k+1,False)*2
print(ans)