#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>
using namespace std;
typedef pair<int, int> pii;
#define rep(i,x) for(int i=0;i<(int)(x);i++)

int main(){
	long long int r, c, ans;
	cin >> r >> c;
	if (r==c) cout << (r*c-1)/4 << endl;
	else cout << (r*c-1)/2 << endl;
	return 0;
}