#!/usr/bin/env python3
n, i, k = map(int, input().split(" "))

nk = n**k

ans = (i-1)**(k+1)

sum = 0
for j in range(i, n):
    sum = sum + (j**k)

ans = (ans - sum) / (2 * nk)

print(ans + (n+1)/2)