from scipy.special import comb

f,n,k=map(int,input().split())
ex=0
for i in range(f):
    p_i = comb(n,k)*((f-k+1)/f)**(k)*(k/f)**(n-k) -  comb(n,k)*((f-k)/f)**(k)*(k-1/f)**(n-k)
    ex+=(i+1)*pi
print(ex)