#include<iostream>
#include<cmath>
using namespace std;
int main() {
	long long int x,y,Y;
	cin >> y >> x;
	Y = (y+1.9)/2;
	if(y%2==1){
		 cout << x*Y-long (x+1.9)/2 << endl;
	}
	else{
		cout << x*Y-1 << endl;
	}
	return 0;
}