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

D = [d // 2 - 1 for d in D]
ans = 1
k = D.index(min(D))
D = D[k:] + D[:k]
ma_d = max(D)
ma = 0
for i in range(D[0] + 1):
    l = 0
    r = ma_d + 1

    def ok(x):
        y = D[0] - i
        for d in D[1:]:
            if y + d < x:
                return False
            y = d - max(0, x - y)

        return y + i >= x

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

    ma = max(ma, l)

ans += ma
print(ans * 2)