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

int main(){
    ll R,C;
    cin >> R >> C;

    ll res = 0;

    if(R == C){
        if(R%2==0){
            res = R*C/4;
        } else {
            res = (R*C-1)/4+1;
        }
    } else {
        res = (R*C-1)/2+1;
    }
    cout << res-1 << endl;
    return 0;
}