#include <bits/stdc++.h>
#define rep(i,j,k) for(int i=(int)j;i<(int)k;i++)
#define itrep(i,x) for(auto i=(x).begin(); i!=(x).end();i++)
#define Sort(x) sort((x).begin(),(x).end())
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define vi vector<int>
#define INF (int)1e9
#define INFL 1e18
#define MOD 1000000007
#define pb push_back
#define MP make_pair
#define PI 3.1415926535
typedef long long int ll;
typedef std::pair<int,int> P;
int D=1;
int dx[4]={0,1,0,-1},dy[4]={-1,0,1,0};

using namespace std;


int main(){
    ll r,c;
    cin>>r>>c;
    
    if(r!=c){
        if(r%2==0)cout<<r/2*c-1<<endl;
        else {
            if(c%2==0)cout<<r/2*c+c/2-1<<endl;
            else cout<<r/2*c+c/2+1-1<<endl;
        }
    }else {
        if(r%2==0 || c%2==0)cout<<r*c/4-1<<endl;
        else cout<<r*c/4<<endl;
    }
    
    
    return 0;
}