#include <bits/stdc++.h>

using namespace std;

typedef pair<int,int> P;
typedef pair<int,pair<int,int>> PP;
typedef long long ll;

const double EPS = 1e-8;
const int INF = 1e9;
const int MOD = 1e9+7;

int dy[] = {0,1,0,-1};
int dx[] = {1,0,-1,0};

int main(void) {
	ll r,c;
	cin >> r >> c;

	if(r!=c){
		if(r<c)swap(r,c);
		if(r%2==0)
			cout << r/2*c-1;
		else
			cout << r/2*c+(c+1)/2-1;
	}else{
		cout << (r*c-1)/4;
	}

	return 0;
}