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

Combi=[[] for i in range(N+1)]
Combi[0]=[1,0]

for i in range(1,N+1):
    Combi[i].append(1)
    for j in range(i):
        Combi[i].append(Combi[i-1][j]+Combi[i-1][j+1])
    Combi[i].append(0)


LIST=[0]*(F+1)
S=0

for i in range(F,0,-1):
    ANS=0
    up=F-i+1
    #print(i,up,F-up)
    for j in range(K,N+1):
        ANS+=Combi[N][j]*pow(up,j)*pow(F-up,N-j)
    LIST[i]=ANS-S
    S+=LIST[i]
    

S=0
for i in range(len(LIST)):
    S+=i*LIST[i]

print(S/sum(LIST))