import java.io.BufferedReader; import java.io.FileNotFoundException; import java.io.IOException; import java.io.InputStreamReader; import java.util.ArrayList; import java.util.Arrays; import java.util.Collections; import java.util.Comparator; import java.util.Iterator; import java.util.LinkedList; import java.util.List; import java.util.TreeSet; public class Main { // public static int w; // public static int k; // public static int n; // public static int[][] dp; // public static int[] r; // public static long[] buy; // public static List list = new ArrayList(); // public static NodeW[] product; // public static int[][] dists; // public static int x; // public static int[][] memo; // public static String str; // public static HashSet set; public static void main(String[] args) throws NumberFormatException, IOException{ ContestScanner in = new ContestScanner(); long n = in.nextLong(); long m = in.nextLong(); // long n2 = n; // long m2 = m; // if(n%2 == 1){ // n++; // } // if(m%2 == 1){ // m++; // } if(n*m/2-1 < 0) System.out.println(0); else System.out.println(n*m/2-1); } } class MyComp implements Comparator{ public int compare(int[] a, int[] b) { return a[0] - b[0]; } } class Reverse implements Comparator{ public int compare(Integer arg0, Integer arg1) { return arg1 - arg0; } } class Node{ int id; List edge = new ArrayList(); public Node(int id){ this.id = id; } public void createEdge(Node node){ edge.add(node); } } class NodeW{ int id; // int dist; List edge = new ArrayList(); List costList = new ArrayList(); public NodeW(int id) { this.id = id; } public void createEdge(NodeW node, int cost) { edge.add(node); costList.add(cost); // node.setDist(dist+1); } // public void setDist(int dist){ // this.dist = Math.max(this.dist, dist); // for(NodeW node: edge){ // node.setDist(dist+1); // } // } } class Range> implements Comparable>{ T start; T end; public Range(T start, T end){ this.start = start; this.end = end; } public boolean inRange(T val){ if(start.compareTo(val) <= 0 && end.compareTo(val) >= 0){ return true; } return false; } public boolean isCommon(Range range){ if(inRange(range.start) || inRange(range.end) || range.inRange(start)){ return true; } return false; } public Range connect(Range range){ if(!isCommon(range)) return null; Range res = new Range(start.compareTo(range.start) <= 0 ? start : range.start, end.compareTo(range.end) >= 0 ? end : range.end); return res; } public boolean connectToThis(Range range){ if(!isCommon(range)) return false; start = start.compareTo(range.start) <= 0 ? start : range.start; end = end.compareTo(range.end) >= 0 ? end : range.end; return true; } @Override public int compareTo(Range range) { int res = start.compareTo(range.start); if(res == 0) return end.compareTo(range.end); return res; } public String toString(){ return "["+start+","+end+"]"; } } class RangeSet>{ TreeSet> ranges = new TreeSet>(); public void add(Range range){ Range con = ranges.floor(range); if(con != null){ if(con.connectToThis(range)) range = con; } con = ranges.ceiling(range); while(con != null && range.connectToThis(con)){ ranges.remove(con); con = ranges.ceiling(range); } ranges.add(range); } public String toString(){ StringBuilder bld = new StringBuilder(); for(Range r: ranges){ bld.append(r+"\n"); } return bld.toString(); } } class MyMath{ public static long fact(long n){ long res = 1; while(n > 0){ res *= n--; } return res; } public static long[][] pascalT(int n){ long[][] tri = new long[n][]; for(int i=0; i