#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)

using namespace std;
using ll = long long;

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