#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
int main()
{
    LL r, c;
    scanf("%lld%lld", &r, &c);
    LL k;
    if (r == c)
    {
        if (r % 2 == 1)
        {
            k = (r * r - 1) / 4;
        }
        else
        {
            k = r * r / 4 - 1;
        }
    }
    else if (r % 2 == 1 && c % 2 == 1)
    {
        k = (r * c - 1) / 2;
    } 
    else
    {
        k = r * c / 2 - 1;
    }
    printf("%lld\n", k);
    return 0;
}