```go
package main

import (
	"fmt"
	"sort"
)

func main() {
	var n, k int64
	fmt.Scan(&n, &k)
	var divisors []int64
	for i := int64(1); i*i <= n; i++ {
		if n % i == 0 {
			divisors = append(divisors, i)
			if i != n/i {
				divisors = append(divisors, n/i)
			}
		}
	}
	sort.Slice(divisors, func(a, b int) bool {
		return divisors[a] < divisors[b]
	})
	if int64(len(divisors)) < k {
		fmt.Println(-1)
	} else {
		fmt.Println(divisors[k-1])
	}
}
```