#include<bits/stdc++.h>
using namespace std;

int main(){
    long long r,c,k;
    cin >> r >> c;
    if(r!=c) k=r*c/2;
    else k=r*c/4;
    if(r%2==0||c%2==0) k--;
    if(r==1&&c==1) k=0;
    cout << k << endl;

    return 0;
}