#include<iostream>
#include<iomanip>
#include<string>
#include<algorithm>
#include<vector>
#include<set>
#include<list>
#include<queue>
#include<math.h>
#include<bitset>
using ll = long long;
using namespace std;

int main(){
    ll r, c, x = 0, r1, c1;
    cin >> r >> c;
    r1 = r, c1 = c;
    if (r%2) x = c;
    r /= 2;
    if (c%2) x = (x+1)/2;
    else x /= 2;

    if (r1 != c1) cout << r*c -1 + x << endl;
    else {
        c /= 2;
        if (!r && !c) cout << 0 << endl;
        else if (!(c%2)) cout << r*c - 1 << endl;
        else cout << r*c - 1 + x << endl;
    }
}