#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    ll R,C; cin >> R >> C;
    if(R == C) {
        cout << (R * C + 3) / 4 - 1 << "\n";
    } else {
        cout << (R * C + 1) / 2 - 1 << "\n";
    }
}