import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n, k = na()

a = na()
f = 10 ** 18
fi = -1
for i in range(k):
    if (a[(i+1) % k] - a[i]) % (2 * n) < f:
        f = (a[(i+1) % k] - a[i]) % (2 * n)
        fi = i

x = a[fi]
a = [(a[i] - x) % (2 * n) // 2 for i in range(k)]
a.sort()
a.append(n)


ok = 1
ng = n + 1

while ng - ok > 1:
    mid = (ok + ng) // 2
    
    FLAG = 0
    for x in range(a[1]):
        y = x
        for i in range(1, k):
            y = max(y + mid, a[i])
            if y >= a[i+1]:
                break
        else:
            if y <= x + n:
                FLAG = 1
            break
    if FLAG:
        ok = mid
    else:
        ng = mid

print(ok * 2)