n, m = map(int, input().split())
b = list(map(int, input().split()))
s = n * m - sum(b)
ans = n - 1
for i in range(m - 1):
    s -= n - b[i]
    ans = min(ans, b[0] - 1 + (m + s) / (m - (i + 1)))
print(ans)