#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    /* nter your code here. Read input from STDIN. Print output to STDOUT */   
    int n, m;
    cin >> n >> m;
    long long p = n * 1LL * m;
    if(p % 2 == 0) {
        cout << p - 1 << endl;
    } else {
          cout << p / 2 + 1 << '\n';
    }
    return 0;
}