import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.Deque;
import java.util.HashMap;
import java.util.PriorityQueue;
import java.util.StringTokenizer;


class Main{

static final PrintWriter out=new PrintWriter(System.out);
static final int INF=Integer.MAX_VALUE/2;

public static void main(String[] args) throws IOException{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
String line="";
while((line=br.readLine())!=null&&!line.isEmpty()){
StringTokenizer st=new StringTokenizer(line);
int a=Integer.parseInt(st.nextToken());
int b=Integer.parseInt(st.nextToken());
if(a%2==0||b%2==0){
if(a==b) out.println(a*b/4-1);
else out.println(a*b/2-1);
}
else{
if(a==b) out.println((a*b-1)/4);
else out.println((a*b-1)/2);
}
out.flush();
}
}

static class Pair{
int x;
int y;
Pair(int x,int y){
this.x=x; this.y=y;
}
}

static class Edge{
int from;
int to;
int cost;
Edge(int from,int to,int cost){
this.from=from; this.to=to; this.cost=cost;
}
Edge(int to,int cost){
this.to=to; this.cost=cost;
}
}

static class Node{
int d;
int v;
Node(int d,int v){
this.d=d; this.v=v;
}
final static Comparator<Node> DISTANCE_ORDER=new DistanceOrderComparator();
static class DistanceOrderComparator implements Comparator<Node>{
public int compare(Node n1,Node n2){
return (n1.d>n2.d)?1:(n1.d<n2.d)?-1:0;
}
}
}
}