#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using ll = long long;

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