#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    long long R,C;
    cin >> R >> C;
    
    if ( R == C ) { cout <<  (R*C)/4 + (R%2 ? 1 : 0) - 1 << endl;  }
    else { cout << ( (R*C+1)/2 - 1 ) << endl; }
    
    return 0;
}