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