#include <iostream>
using namespace std;
typedef signed long long ll;
 
#define REP(i,n) for (int i = 0; i < (n); i++)
#define RREP(i,n) for (int i = (n)-1; i >= 0; i--)
#define FOR(i,s,n) for (int i = (s); i < (n); i++)
#define RFOR(i,s,n) for (int i = (n)-1; i >= (s); i--)
 
 
int main() {
	
	ll r,c;
	cin>> r >> c;
	ll total = r*c;
	
	if(r%2==1&&c%2==1)total--;
	
	if(r == c)total /= 4;
	else total /= 2;
	
		if(r%2==1&&c%2==1)total++;
	
	
	cout << total-1<< endl;  
	
 
	return 0;
}