#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

#define p(s) cout<<(s)<<endl
#define REP(i,n,N) for(ll i=n;i<N;i++)
#define RREP(i,n,N) for(ll i=N-1;i>=n;i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define F first
#define S second
typedef long long int ll;
using namespace std;
const ll inf=1e9+7;

ll R,C;

int main(){
    cin>>R>>C;
    if(R!=C) p(R*C/2-(R*C%2==0));
    else p(R*C/4-(R*C%2==0));

    return 0;
}