#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

int main()
{
    long long r, c;
    cin >> r >> c;

    long long ret;
    if(r == c){
        if(r % 2 == 0)
            ret = (r / 2) * (r / 2);
        else
            ret = (r / 2) * ((r + 1) / 2) + 1;
    }
    else{
        ret = (r * c + 1) / 2;
    }

    -- ret;
    cout << ret << endl;

    return 0;
}