source_code: import java.io.InputStream;
  import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.BufferedWriter;
  import java.io.Writer;
  import java.io.OutputStreamWriter;
  import java.util.HashMap;
  import java.io.IOException;
  import java.util.InputMismatchException;
  import java.io.PrintWriter;
  import java.io.OutputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   *
   * @author Alex
   */
  public class Main {
      public static void main(String[] args) {
          InputStream inputStream = System.in;
          OutputStream outputStream = System.out;
          InputReader in = new InputReader(inputStream);
          OutputWriter out = new OutputWriter(outputStream);
          TaskC solver = new TaskC();
          solver.solve(1, in, out);
          out.close();
      }
      static class TaskC {
          int BAD = 11111;
          int rows;
          int cols;
          HashMap<IntIntPair, Integer>[] mem;
  
          boolean contains(int mem, int index) {
              if(index < 0) return false;
              return ((mem >> index) & 1) == 1;
          }
  
          int add(int mem, int index) {
              if(((mem >> index) & 1) == 0) {
                  mem += (1 << index);
              }
              return mem;
          }
  
          int size(int mem) {
              int res = 0;
              while(mem > 0) {
                  if(mem % 2 == 1) res++;
                  mem /= 2;
              }
              return res;
          }
  
          void test() {
              if(contains(5, 0) == false) throw new RuntimeException();
              if(contains(5, 1) == true) throw new RuntimeException();
              if(contains(5, -1) == true) throw new RuntimeException();
              if(contains(5, 2) == false) throw new RuntimeException();
              if(contains(5, 3) == true) throw new RuntimeException();
              if(add(0, 2) != 4) throw new RuntimeException();
              if(add(4, 0) != 5) throw new RuntimeException();
              if(add(5, 0) != 5) throw new RuntimeException();
              if(size(5) != 2) throw new RuntimeException();
          }
  
          int dp(int row, int remabove, int squareabove) {
              if(row == rows) {
                  if(remabove == 0) return 0;
                  return BAD;
              }
              if(mem[row].containsKey(new IntIntPair(remabove, squareabove)))
                  return mem[row].get(new IntIntPair(remabove, squareabove));
              int res = BAD;
              int possibilities = 1 << cols;
              for(int poss = 0; poss < possibilities; poss++) {
                  int have = 0;
                  for(int j = 0; j < cols; j++)
                      if(((poss >> j) & 1) == 1) {
                          have += 1 << j;
                      }
                  boolean works = true;
                  for(int above = 0; above < cols; above++)
                      if(((remabove >> above) & 1) == 1) {
                          if(((have >> above) & 1) == 0) {
                              works = false;
                              break;
                          }
                      }
                  if(works) {
                      int remhere = 0;
                      for(int j = 0; j < cols; j++) {
                          if(!contains(have, j - 1) && !contains(have, j) && !contains(have, j + 1) && !contains(squareabove, j)) {
                              remhere = add(remhere, j);
                          }
                      }
                      res = Math.min(res, size(have) + dp(row + 1, remhere, have));
                  }
              }
              mem[row].put(new IntIntPair(remabove, squareabove), res);
              return res;
          }
  
          public void solve(int testNumber, InputReader in, OutputWriter out) {
              test();
              int n = in.readInt(), m = in.readInt();
              cols = Math.min(n, m);
              rows = Math.max(n, m);
              mem = new HashMap[rows];
              for(int i = 0; i < mem.length; i++) mem[i] = new HashMap<>();
              int res = dp(0, 0, 0);
              out.printLine(cols * rows - res);
          }
  
      }
  
      static class IntIntPair implements Comparable<IntIntPair> {
          public final int first;
          public final int second;
  
          public IntIntPair(int first, int second) {
              this.first = first;
              this.second = second;
          }
  
  
          public boolean equals(Object o) {
              if(this == o) return true;
              if(o == null || getClass() != o.getClass()) return false;
  
              IntIntPair pair = (IntIntPair) o;
  
              return first == pair.first && second == pair.second;
  
          }
  
  
          public int hashCode() {
              int result = Integer.hashCode(first);
              result = 31 * result + Integer.hashCode(second);
              return result;
          }
  
  
          public String toString() {
              return "(" + first + "," + second + ")";
          }
  
          @SuppressWarnings({"unchecked"})
          public int compareTo(IntIntPair o) {
              int value = Integer.compare(first, o.first);
              if(value != 0) {
                  return value;
              }
              return Integer.compare(second, o.second);
          }
  
      }
  
      static class InputReader {
          private InputStream stream;
          private byte[] buf = new byte[1024];
          private int curChar;
          private int numChars;
          private SpaceCharFilter filter;
  
          public InputReader(InputStream stream) {
              this.stream = stream;
          }
  
          public int read() {
              if(numChars == -1)
                  throw new InputMismatchException();
              if(curChar >= numChars) {
                  curChar = 0;
                  try {
                      numChars = stream.read(buf);
                  } catch(IOException e) {
                      throw new InputMismatchException();
                  }
                  if(numChars <= 0)
                      return -1;
              }
              return buf[curChar++];
          }
  
          public int readInt() {
              int c = read();
              while(isSpaceChar(c))
                  c = read();
              int sgn = 1;
              if(c == '-') {
                  sgn = -1;
                  c = read();
              }
              int res = 0;
              do {
                  if(c < '0' || c > '9')
                      throw new InputMismatchException();
                  res *= 10;
                  res += c - '0';
                  c = read();
              } while(!isSpaceChar(c));
              return res * sgn;
          }
  
          public boolean isSpaceChar(int c) {
              if(filter != null)
                  return filter.isSpaceChar(c);
              return isWhitespace(c);
          }
  
          public static boolean isWhitespace(int c) {
              return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
          }
  
          public interface SpaceCharFilter {
              public boolean isSpaceChar(int ch);
  
          }
  
      }
  
      static class OutputWriter {
          private final PrintWriter writer;
  
          public OutputWriter(OutputStream outputStream) {
              writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
          }
  
          public OutputWriter(Writer writer) {
              this.writer = new PrintWriter(writer);
          }
  
          public void close() {
              writer.close();
          }
  
          public void printLine(int i) {
              writer.println(i);
          }
  
      }
  }
  
  