#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <map>
#include <functional>
#include <set>
#include <numeric>
#include <stack>
#include <utility>
#include <time.h>
//#include "util.h"

using namespace std;
typedef long long ll;
typedef unsigned long long ull;

#define PI 3.14159265358979323846
#define EPS 1e-6
#define MIN(a,b) ((a)<(b)?(a):(b))
#define MAX(a,b) ((a)>(b)?(a):(b))
#define CHAR_BIT 8

template <typename _Ty>
ostream& operator << (ostream& ostr, const vector<_Ty>& v) {
	if (v.empty()) {
		cout << "{ }";
		return ostr;
	}
	cout << "{" << v.front();
	for (auto itr = ++v.begin(); itr != v.end(); itr++) {
		cout << ", " << *itr;
	}
	cout << "}";
	return ostr;
}

int yuki0084()
{
	ll R, C;
	cin >> R >> C;

	if (R == C) {
		if (R * C % 2)
			cout << R * C / 4;
		else
			cout << R * C / 4 - 1;
	}
	else {
		if (R * C % 2)
			cout << R * C / 2;
		else
			cout << R * C / 2 - 1;
	}
	cout << endl;

	return 0;
}

int main()
{
	//clock_t start, end;
	//start = clock();

	yuki0084();

	//end = clock();
	//printf("%d msec.\n", end - start);

	return 0;
}