#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<algorithm>
#include<vector>
#include<math.h>
#include<iomanip>
#include<stack>
#include<queue>
#include<set>
#include<map>

using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<int , int>;

//const ll mod = 1000000007;
const ll mod = 998244353;
const ld pi = 3.14159265358979;

int main() {
	//ios::sync_with_stdio(false);
	//cin.tie(0);
	//cout << fixed << setprecision(15);
	ll r , c;
	cin >> r >> c;
	ll ans = r * c;
	ans = (ans + 1) / 2;
	cout << ans - 1 << endl;
	return 0;
}