#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cctype>
#include<math.h>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#include<set>
#include<map>
#include<stdlib.h>
#include<iomanip>

using namespace std;

#define ll long long
#define ld long double
#define EPS 0.0000000001
#define INF 1e9
#define LINF (ll)INF*INF
#define MOD 1000000007
#define rep(i,n) for(int i=0;i<(n);i++)
#define loop(i,a,n) for(int i=a;i<(n);i++)
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)

#define int ll //!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int,int> pii;
typedef vector<pii> vp;

int gcd(int a, int b){
    if(b==0) return a;
    return gcd(b,a%b);
}
int lcm(int a, int b){
    return a*b/gcd(a,b);
}


signed main(void) {
    int n,m;
    cin >> n >> m;
    int ans;
    if(n%2 && m%2){
        if(n == m) ans = n/2 * (n+1)/2 + 1;
        else ans = n/2 * m + m/2 + 1;
    }else if(n%2){
        ans = m/2 * n;
    }else if(m%2){
        ans = n/2 * m;
    }else{
        if(n == m)ans = n/2 * m/2;
        else ans = n/2 * m;
    }
    cout << ans - 1 << endl;
}