#include <bits/stdc++.h>
using namespace std;
int main() {
    long long a, b, c;
    cin >> a >> b;
    if (a == b) {
        if (a % 2 != 0) {
            c = (a * b - 1) / 4;
        } else {
            c = a * b / 4 - 1;
        }
    } else {
        if (a % 2 != 0 && b % 2 != 0) {
            c = (a * b - 1) / 2;
        } else {
            c = a * b / 2 - 1;
        }
    }
    cout << c << endl;
    return 0;
}