#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
    ll r,c;cin >> r >> c;
    if(r%2==0&&c%2==0){
        cout << (r/2)*(c/2)*2-1 << endl;
    }
    else if(r%2==1&&c%2==1){
        cout << 1+r/2+c/2+(r/2)*(c/2)*2-1 << endl;
    }
    else{
        if(r%2==1){
            swap(r,c);
        }
        cout << r/2+(r/2)*(c/2)*2-1 << endl;
    }
}