package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

const MaxInt = math.MaxInt64

var scanner *bufio.Scanner

func main() {
	n, k := Int(), Int()
	ans := -1
	if n >= k {
		ans = k - 1
	}
	fmt.Println(ans)
}

func init() {
	scanner = bufio.NewScanner(os.Stdin)
	scanner.Buffer([]byte{}, MaxInt)
	scanner.Split(bufio.ScanWords)
}

func Int() int {
	scanner.Scan()
	ret, _ := strconv.ParseInt(scanner.Text(), 10, 0)
	return int(ret)
}

func Ints(n int) []int {
	ret := make([]int, n)
	for i := 0; i < n; i++ {
		ret[i] = Int()
	}
	return ret
}

func Max(a, b int) int {
	if a < b {
		return b
	}
	return a
}

func Min(a, b int) int {
	if a < b {
		return a
	}
	return b
}