n,k = map(int, input().split())
a = list(map(int, input().split()))
a.append(a[0] + 2*n)

an = a.copy()

for i in range(k+1):
    an[i] -= a[0]

ans = 0
for s in range(1, an[1], 2):
    ok = 0
    ng = n
    while ng - ok > 1:
        mid = (ok + ng) // 2
        def check():
            x = 2*mid
            si = s
            for i in range(k-1):
                if si + x > an[i+2]: return False
                si = max(si+x, an[i+1]+1)
            return si + x <= s + 2*n
        if check(): ok = mid
        else: ng = mid
    ans = max(ans, ok*2)
print(ans)