func E(k int64) float64 {
if k == 0 {
return 0
}
e := cache[k]
if e != 0 {
return e
}
M := float64(1)
for i := k - 1; i >= 0; i-- {
M += float64(mathutil.Choose(k, i)) * (1 + E(i))
}
ret := M / (math.Pow(2, float64(k)) - 1)
cache[k] = ret
return ret
}
func main() {
fmt.Printf("%v\n", 7*(1-float64(mathutil.Choose(60, 20))/float64(mathutil.Choose(70, 20))))
}