## https://yukicoder.me/problems/no/1478

def main():
    N, M = map(int, input().split())

    B = list(map(int, input().split()))


    exp = [0] * M
    cum_exp = [0] * M
    cum_e = 0
    for i in reversed(range(M)):
        e1 = N - B[i]
        if i == M - 1:
            exp[i] = e1
        else:
            r = (i + 1) / M
            ans = 1 + cum_exp[i + 1] / M
            ans *= 1 / (1 - r)
            exp[i] = min(exp[i + 1] + B[i + 1] - B[i], ans)

        cum_e += exp[i]
        cum_exp[i] = cum_e

    
    answer = B[0] - 1 + exp[0]
    print(answer)






        














if __name__ == "__main__":
    main()