import math

def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx]); idx +=1
    S = int(input[idx]); idx +=1
    writer_id = int(input[idx]); idx +=1
    a = list(map(int, input[idx:idx+N]))
    idx +=N
    
    a_w = a[writer_id]
    A_author = a_w + 100 * S
    m = N - 1  # number of non-author participants
    
    lower_list = []
    for i in range(N):
        if i == writer_id:
            continue
        a_i = a[i]
        RHS_i = a_w - a_i + 50 * S + 1
        if RHS_i <= 0:
            print(0.0)
            return
        x_i = (250 * S) / RHS_i - 4
        lower_i = math.floor(x_i) + 1
        lower_i = max(lower_i, 1)
        if lower_i > m:
            print(0.0)
            return
        lower_list.append(lower_i)
    
    lower_list.sort()
    # Check if all lower_list[i] <= i+1 (i is 0-based)
    for i in range(len(lower_list)):
        if lower_list[i] > (i + 1):
            print(0.0)
            return
    
    prob = 1.0
    for i in range(len(lower_list)):
        numerator = (i + 2) - lower_list[i]
        denominator = i + 1  # i is 0-based, denominator is (i+1)
        prob *= numerator / denominator
    
    print("{0:.12f}".format(prob))
    
if __name__ == '__main__':
    main()