#include <bits/stdc++.h>
#define For(i, a, b) for(long long i = a; i < b; i++)
#define rep(i, n) For(i, 0, n)
#define rFor(i, a, b) for(long long i = a; i >= b; i--)
#define ALL(v) (v).begin(), (v).end()
#define rALL(v) (v).rbegin(), (v).rend()
using namespace std;

using lint = long long;
using ld = long double;

int main() {
    lint r, c;
    cin >> r >> c;
    if (r == c) {
        cout << (r % 2 == 0 ? r * c / 4 - 1 : r * c / 4) << endl;
    } else {
        cout << (r * c % 2 == 0 ? r * c / 2 - 1 : r * c / 2) << endl;
    }
}