F, N, K = map(int,input().split())
import math

dp = [0] * (F + 1)
for x in range(1, F + 1):
    for t in range(K, N + 1):
        p = (F-x+1)/F
        q = ((x-1)/F)
        s = p**t * q**(N-t) * math.comb(N, t)
        dp[x] += s
ans = 0
for i in range(1, F):
    dp[i] -= dp[i + 1]
for it in range(1, F + 1):
    ans += it * dp[it]
print(ans)