package main

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

func main() {
	in := bufio.NewReader(os.Stdin)
	var n, k int64
	fmt.Fscan(in, &n, &k)

	l := int64(1)
	if k-n > l {
		l = k - n
	}
	u := (k - 1) / 2
	if u > n {
		u = n
	}

	if u < l {
		fmt.Println(0)
	} else {
		fmt.Println(u - l + 1)
	}
}