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]
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
        if y >= x:
            y = d
        else:
            y = d - (x - y)
    return y >= 0


l = 0
r = max(D) + 1
while r - l > 1:
    mid = (l + r) // 2
    if ok(mid):
        l = mid
    else:
        r = mid

ans += l
if ans % 2 == 1:
    ans -= 1

print(ans)