package main

import (
	"bufio"
	"fmt"
	"os"
)

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

func main() {
	in := bufio.NewReader(os.Stdin)
	var n, m int
	fmt.Fscan(in, &n, &m)

	if m == 0 {
		fmt.Println(1)
		return
	}
	if m == n {
		fmt.Println(0)
		return
	}
	fmt.Println(min(m, n-m))
}