Tanechka is shopping in the toy shop. There are exactly n toys in the shop for sale, the cost of the i-th toy is i burles. She wants to choose two toys in such a way that their total cost is k burles. How many ways to do that does she have?

Each toy appears in the shop exactly once. Pairs (a, b) and (b, a) are considered equal. Pairs (a, b), where a=b, are not allowed.

Input

The first line of the input contains two integers n, k (1 ≤ n, k ≤ 10^{14}) — the number of toys and the expected total cost of the pair of toys.

Output

Print the number of ways to choose the pair of toys satisfying the condition above. Print 0, if Tanechka can choose no pair of toys in such a way that their total cost is k burles.

Examples

Input

8 5


Output

2


Input

8 15


Output

1


Input

7 20


Output

0


Input

1000000000000 1000000000001


Output

500000000000

Note

In the first example Tanechka can choose the pair of toys (1, 4) or the pair of toys (2, 3).

In the second example Tanechka can choose only the pair of toys (7, 8).

In the third example choosing any pair of toys will lead to the total cost less than 20. So the answer is 0.

In the fourth example she can choose the following pairs: (1, 1000000000000), (2, 999999999999), (3, 999999999998), ..., (500000000000, 500000000001). The number of such pairs is exactly 500000000000.
                            /* Rajkin Hossain */
import java.io.*;
import java.util.*;

public class A {

    //static FastInput k = new FastInput("/home/rajkin/Desktop/input.txt");
    static FastInput k = new FastInput(System.in);
    static FastOutput z = new FastOutput();
    
    static long N,K;
    
    static void startAlgorithm() {
    	if(N == 1 || K == 1) {
    		z.println(0);
    	}
    	else if(N < K) {
    		long right = K-1;
    		long total = right / 2;
    		long right2 = right - (N+1) + 1;
    		z.println(total - right2 < 0 ? 0 : total - right2);
    	}
    	else if(N >= K) {
    		z.println((K-1) / 2);
    	}
    }
	
	static void getInputsAndInitializes() {
		N = k.nextLong();
		K = k.nextLong();
    }
    
	public static void main(String [] args) throws FileNotFoundException {        
		while(k.hasNext()) {
        	getInputsAndInitializes();
        	startAlgorithm();
        }
		
        z.flush();
        System.exit(0);
	}
	
	
	/* MARK: Pair implementation */
	
	public static class Pair{
		int current, target;
		public Pair(int current, int target) {
			this.current = current;
			this.target = target;
		}
	}
	
	
	/* MARK: SetBit implementation */
	
	public static long setBit(long pos, long value) {
		return value |= (1l << pos);
	}
	
    /* MARK: FastInput and FastOutput implementation */
	 
    public static class FastInput {
        BufferedReader reader;
        StringTokenizer tokenizer;
 
        public FastInput(InputStream stream){
        	reader = new BufferedReader(new InputStreamReader(stream));
        }
        
	    public FastInput(String path){
	        try {
	        	reader = new BufferedReader(new FileReader(path));
			} catch (FileNotFoundException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
	        tokenizer = null;
	    }
 
        public String next() {
            return nextToken();
        }
        
        public String nextLine() {
            try {
				return reader.readLine();
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
            
            return null;
        }
 
        public boolean hasNext(){
            try {
                return reader.ready();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return false;
        }
 
        public String nextToken() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                String line = null;
                try {
                    line = reader.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
                if (line == null) {
                    return null;
                }
                tokenizer = new StringTokenizer(line);
            }
            return tokenizer.nextToken();
        }
 
        public Integer nextInt() {
            return Integer.parseInt(nextToken());
        }
       
        public Long nextLong() {
            return Long.valueOf(nextToken());
        }
    	
    	public void inputIntegerList(ArrayList<Integer> list, int size) {
    		for(int i = 0; i<size; i++) {
    			list.add(nextInt());
    		}
    	}
    	
    	public void inputLongList(ArrayList<Long> list, int size) {
    		for(int i = 0; i<size; i++) {
    			list.add(nextLong());
    		}
    	}
    	
    	public void inputWordList(ArrayList<String> list, int size) {
    		for(int i = 0; i<size; i++) {
    			list.add(next());
    		}
    	}
    	
    	public void inputLineList(ArrayList<String> list, int size) {
    		for(int i = 0; i<size; i++) {
    			list.add(nextLine());
    		}
    	}
    	
    	public void inputIntegerArray(int [] array, int size) {
    		for(int i = 0; i<size; i++) {
    			array[i] = nextInt();
    		}
    	}

    	public void inputLongArray(long [] array, int size) {
    		for(int i = 0; i<size; i++) {
    			array[i] = nextLong();
    		}
    	}
    	
    	public void inputWordArray(String [] array, int size) {
    		for(int i = 0; i<size; i++) {
    			array[i] = next();
    		}
    	}
    	
    	public void inputLineArray(String [] array, int size) {
    		for(int i = 0; i<size; i++) {
    			array[i] = nextLine();
    		}
    	}
    }
 
    public static class FastOutput extends PrintWriter {
    	public FastOutput() { 
            super(new BufferedOutputStream(System.out)); 
    	}
    	
    	public void printSideBySideSet(Set<Object> set, int size) {
    		boolean isFirstElementPrinted = false;
    		for(Object element : set) {
    			if(!isFirstElementPrinted) {
    				print(element);
    				isFirstElementPrinted = true;
    				
    				return;
    			}
    			print(" "+element);
    		}
    		println();
    	}
    	
    	public void printLineByLineSet(Set<Object> set, int size) {
    		for(Object element : set) {
    			println(element);
    		}
    	}
    	
    	public void printSideBySideList(ArrayList<Object> list, int size) {
    		print(list.get(0));
    		for(int i = 1; i<size; i++) {
    			print(" "+list.get(i));
    		}
    		println();
    	}
    	
    	public void printLineByLineList(ArrayList<Object> list, int size) {
    		for(int i = 0; i<size; i++) {
    			println(list.get(i));
    		}
    	}
    	
    	public void printSideBySideArray(Object [] array, int size) {
    		print(array[0]);
    		for(int i = 1; i<size; i++) {
    			print(" "+array[i]);
    		}
    		println();
    	}
    	
    	public void printLineByLineArray(Object [] array, int size) {
    		for(int i = 0; i<size; i++) {
    			println(array[i]);
    		}
    	}
    }
}