#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    ll r,c; cin >> r >> c;
    if(r==c){
        if(r*c%2==0)printf("%lld\n",r*c/4-1);
        else printf("%lld\n",r*c/4);
    }
    else{
        if(r*c%2==0) printf("%lld\n",r*c/2-1);
        else printf("%lld\n",r*c/2);
    }
}