#include <bits/stdc++.h>
using namespace std;

int main(){
	int n,m;
	cin>>n>>m;
	if(n % 2 == 0){
		cout<<n / 2 * m<<endl;
	}else{
		cout<<n / 2 * m + m / 2<<endl;
	}
	
	
	return 0;
}

/**************************************************************
	Problem: 1539
	User: admin
	Language: C++
	Result: Accepted
	Time:56 ms
	Memory:2072 kb
****************************************************************/