INF = 1<<60

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

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

def func(size):
    MAX = -INF
    for i in range(2, K*2):
        MAX = max(MAX, A[i-2]-size*(i-2))
        if A[i]-size*(i-1)-2 < MAX:
            return False
    return True

left = 0
right = N*2//K+1
while left+1 < right:
    mid = (left+right)//2
    if func(mid):
        left = mid
    else:
        right = mid

print(left)