#include <bits/stdc++.h>

using namespace std;

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)
#define unless(p) if(!(p))
#define until(p) while(!(p))

using ll = long long;
using P = std::tuple<int,int>;

const int dx[8] = {-1, 1, 0, 0, -1, -1, 1, 1}, dy[8] = {0, 0, -1, 1, -1, 1, -1, 1};

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

    ll R, C;
    std::cin >> R >> C;

    ll res = 0ll;
    if(R == C){
        if(R == 1){
            res = 1;
        }else{
            res = R * (R - 1) / 2;
        }
    }else if(R % 2 == 0 && C % 2 == 0){
        res = R * C / 2;
    }else if(R % 2 == 1 && C % 2 == 1){
        res = R * (C / 2) + (R / 2) * C - 2ll * (R / 2) * (C / 2) + 1;
    }else{
        res = R * C / 2;
    }

    std::cout << (res - 1) << std::endl;
}