# https://yukicoder.me/problems/no/2869

def main():
    N, S, writer_id = map(int, input().split())
    A = list(map(int, input().split()))

    writer_score = 100 * S + A[writer_id]

    def fn(r: int):
        x = 50 * S + (50 * S) / (0.8 + 0.2 * r)
        return int(x)
    
    array_map = {}
    for i in range(N):
        if i == writer_id:
            continue

        if fn(1) <= writer_score - A[i]:
            if 0 not in array_map:
                array_map[0] = 0
            array_map[0] += 1
            continue

        low = 1
        high = N
        while high - low > 1:
            mid = (high + low) // 2
            if fn(mid) > writer_score - A[i]:
                low = mid
            else:
                high = mid
        if fn(high) > writer_score - A[i]:
            v = high
        else:
            v = low
        if v not in array_map:
            array_map[v] = 0
        array_map[v] += 1
    
    n = N - 1
    b = 0
    array = [0] * (N - 1)
    for i in range(N - 1):
        if i in array_map:
            b += array_map[i]
        
        array[i] = b / n
        b -= 1
        n -= 1

    array.sort(reverse=True)
    answer = 1.0
    for a in array:
        answer *= a
    print(answer)
        


        

        

if __name__ == "__main__":
    main()