source_code: import java.io.BufferedReader;
  import java.io.FileReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.util.Comparator;
  import java.util.Locale;
  import java.util.StringTokenizer;
  import java.util.TreeMap;
  import java.util.Map.Entry;
  
  public class Solution implements Runnable {
  
  	public static void main(String[] args) {
  		(new Thread(new Solution())).start();
  	}
  
  	BufferedReader in;
  	PrintWriter out;
  	StringTokenizer st;
  
  	String nextToken() throws IOException {
  		while (st == null || !st.hasMoreTokens()) {
  			String r = in.readLine();
  			if (r == null) return null;
  			st = new StringTokenizer(r);
  		}
  		return st.nextToken();
  	}
  
  	int nextInt() throws IOException {
  		return Integer.parseInt(nextToken());
  	}
  
  	long nextLong() throws IOException {
  		return Long.parseLong(nextToken());
  	}
  
  	double nextDouble() throws IOException {
  		return Double.parseDouble(nextToken());
  	}
  	
  	int count(int q, int n) {
  		int ans = 0;
  		for (int i = 0; i < n; i++) {
  			if (q % 3 != 1) ans++;
  			q /= 3;
  		}
  		return ans;
  	}
  	
  	int count2(int q, int n) {
  		int ans = 0;
  		for (int i = 0; i < n; i++) {
  			if (q % 3 == 2) ans++;
  			q /= 3;
  		}
  		return ans;
  	}
  	
  	int sz;
  	
  	int[] decode(int q, int n) {
  		int[] res = new int[n];
  		for (int i = 0; i < n; i++) {
  			res[i] = q % 3;
  			q /= 3;
  		}
  		return res;
  	}
  	
  	boolean[][] precalc(int n, int m) {
  		boolean[][] res = new boolean[sz][sz];
  		for (int i = 0; i < sz; i++) {
  			int[] ai = decode(i, n);
  			for (int j = 0; j < sz; j++) {
  				int[] aj = decode(j, n);
  				boolean f = true;
  				for (int k = 0; k < n && f; k++) {
  					if (aj[k] == 0) {
  						f = false;
  						if (k > 0 && aj[k - 1] == 1) f = true;
  						if (k < n - 1 && aj[k + 1] == 1) f = true;
  						if (ai[k] == 1) f = true;
  					}
  					if (f && ai[k] == 2) f = (aj[k] == 1);
  				}
  				res[i][j] = f;
  			}
  		}
  		return res;
  	}
  	
  	void solve() throws Exception {
  		int n = nextInt();
  		int m = nextInt();
  		if (n > m) { int t = n; n = m; m = t; }
  		sz = 1;
  		for (int i = 0; i < n; i++) sz *= 3;
  		boolean[][] a = precalc(n, m);
  		int[][] d = new int[m + 1][sz];
  		Arrays.fill(d[0], -1);
  		d[0][0] = 0;
  		for (int i = 1; i <= m; i++) {
  			Arrays.fill(d[i], -1);
  			for (int j = 0; j < sz; j++) {
  				if (d[i - 1][j] != -1) {
  					for (int k = 0; k < sz; k++) {
  						if (a[j][k]) {
  							d[i][k] = Math.max(d[i][k], d[i - 1][j] + count(k, n));
  						}
  					}
  				}
  			}
  		}
  		int ans = 0;
  		for (int i = 0; i < sz; i++) if (count2(i, n) == 0) ans = Math.max(ans, d[m][i]);
  		out.println(ans);
  	}
  
  	public void run() {
  		Locale.setDefault(Locale.UK);
  		try {
  			in = new BufferedReader(new InputStreamReader(System.in));
  			out = new PrintWriter(System.out);
  //			in = new BufferedReader(new FileReader("knights.in"));
  //			out = new PrintWriter("knights.out");
  			solve();
  		} catch (Exception e) {
  			e.printStackTrace();
  			System.exit(1);
  		} finally {
  			out.flush();
  		}
  	}
  
  }