#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <cmath>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
#include <bitset>
using namespace std;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)
typedef long long ll;

int main(){
  ll a, b;
  cin >> a >> b;
  ll ans;
  if(a==b)
  {
    ans = a - 1;
  }
  else
  {
    if(a%2==0)
    {
      ans = (a / 2) * b - 1;
    }
    else if(b%2==0)
    {
      ans = (b / 2) * a - 1;
    }
    else
    {
      ans = (a / 2) * b - 1;
      ans += (b + 1) / 2;
    }
  }
  cout<<ans<<endl;
  return 0;
}