#include<bits/stdc++.h>
#define Li long long
#define Fasting()  ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define PuB push_back
#define PoB pop_back
#define All(x) x.begin(), x.end()
#define F first
#define S second

using namespace std;
const int N=1e5+10;

int main(){
    Fasting();
    cin>>a>>b;
    cout<<ceil(1.0*a*b/2);
}
