content: import java.io.*;
  import java.math.*;
  import java.text.*;
  import java.util.*;
  import java.util.regex.*;
  
  public class solution {
  
      /*
       * Complete the pageCount function below.
       */
      static int pageCount(int n, int p) {
          /*
           * Write your code here.
           */
          // it will take p/2 turns to reach the pth page from front
          // and (n-p)/2 pages from the back. Take the minimum out of the two
          int ans = Math.min(p / 2, n / 2 - p / 2);
          return ans;
      }
  
      private static final Scanner scanner = new Scanner(System.in);
  
      public static void main(String[] args) throws IOException {
          BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));
  
          int n = scanner.nextInt();
          scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])*");
  
          int p = scanner.nextInt();
          scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])*");
  
          int result = pageCount(n, p);
  
          bufferedWriter.write(String.valueOf(result));
          bufferedWriter.newLine();
  
          bufferedWriter.close();
  
          scanner.close();
      }
  }
  