n, k = map(int, input().split())
A = list(map(int, input().split()))
b = A[-1] - 2 * n
D = []
for a in A:
    D.append(a - b)
    b = a

ans = min(D)
D = [d - ans for d in D]
D = [d // 2 for d in D]
ans //= 2
k = D.index(0)
D = D[k:] + D[:k]
D = D[1:]


def ok(x):
    y = 0
    for d in D:
        if y + d < x:
            return False

        add = max(0, x - y)
        y = d - add

    return y >= 0


l = min(D)
r = max(D) + 1
while r - l > 1:
    mid = (l + r) // 2
    if ok(mid):
        l = mid
    else:
        r = mid

ans += l
print(ans * 2)