import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int p = in.nextInt();
        int first = fromFirst(n,p);
        int last = fromLast(n,p);
        System.out.println(first > last ? last : first);
    }
    public static int fromFirst(int n, int p){
        if(p == 1){
            return 0;
        }else{
            return p % 2 == 0 ? p/2 : (p-1)/2;
        }
    }
    public static int fromLast(int n, int p){
        if(n % 2 == 0){
            if(p % 2 == 0){
                return (n-p)/2;
            }else{
                return (n-(p-1))/2;
            }
        }else if(n-1 == p){
            return 0;
        }else{
            return p % 2 == 0 ? ((n-1)-p)/2 : ((n-1)-(p-1))/2;
        }
    }
}