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

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

    return 0;
}