#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <iomanip>
using namespace std;
#define INF 1e9
#define PI acos(-1)
typedef long long ll;


int main() {
	
	ll ans;
	ll r, c;
	cin >> r >> c;

	
	if (r == c) {
		if (r*c%2==0)ans=(r*c/4-1);
		else ans = (r*c / 4);
	}
	else {
		if (r*c%2==0) ans = r * c / 2 - 1;
		else ans = r * c / 2;//中心点で+1
	}

	cout << ans << endl;
	
	return 0;
}