N, M = map(int, input().split())
B = list(map(int, input().split()))
C = [0] * M
D = [0] * M
E = [0] * M
se = N + B[0] - 1
ans = N - 1
for i in reversed(range(M)):
    C[i] = se - B[i]
    if i == M - 1:
        D[i] = C[i]
    else:
        D[i] = D[i + 1] + C[i]
    ans = min(ans, D[i] / (M - i) + M / (M - i))
print(ans)