count = 0
x = [list(map(str, input().split())) for _ in range(3)]
data = " ".join(x[1])
data = sorted(data.replace(" ",''))

def my_combinstion(a: str, r, result=None, indices=None, index=0):
    
    if result is None:
        result = []
    if indices is None:
        indices = []
        
    if index == r:
        pair = tuple(a[i] for i in indices)
        result.append(pair)
        return 
        
    start = indices[-1] + 1 if indices else 0
    for i in range(start, len(a)):
        my_combinstion(a, r, result, indices +[i], index+1)
        
    return result

    
z = my_combinstion(data, int(x[-1][-1]))
for x in z:
    if 'a' in x:
        count+=1
print(count/len(z))