#include <bits/stdc++.h>
using namespace std;

template<typename T>
istream &operator>>(istream &is,vector<T> &a){
    for(auto &v : a) cin >> v;
    return is;
}
template<typename T>
ostream &operator<<(ostream &os,const vector<T> &a){
    if(a.size() == 0) return os;
    cout << a.at(0);
    for(int i=1; i<a.size(); i++) cout << " " << a.at(i);
    cout << "\n";
    return os;
}


int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    long long R,C; cin >> R >> C;
    if(C%2 == 0) swap(R,C);
    if(R == C){
        if(R%2) cout << (R-1)*(C-1)/4+R/2+1-1 << endl;
        else cout << R*C/4-1 << endl;
    }
    else if(R%2 == 0) cout << R*C/2-1 << endl;
    else cout << R*C/2 << endl;
    
}