N, K = map(int, input().split())
A = list(map(int, input().split()))
A2 = A[:]
N2 = N * 2
N4 = N2 * 2
A2 += [i + N2 for i in A]
A2 += [i + N4 for i in A]
damin = N * 2
for i in range(K):
    if damin > A2[i + 1] - A2[i]:
        damin = A2[i + 1] - A2[i]
        damini = i
start = A2[damini] + 1
end = A2[damini + 1]

def check(x):
    for i in range(start, end, 2):
        now = i
        f = True
        for j in range(damini + 1, damini + K + 1):
            if now + x >= A2[j + 1]:
                f = False
                break
            now = max(now + x, A2[j] + 1)
        if f:
            return True
    return False

lb = 1
ub = N
while ub - lb > 1:
    mid = (ub + lb) // 2
    if check(2 * mid):
        lb = mid
    else:
        ub = mid
print(2 * lb)