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

int main()
{
long long A;
long long R;
long long C;
long long iCnt;

	cin >> R;
	cin >> C;

	if( R == C )
	{
		A = R / 2;
		if( R % 2 == 0 )
  			iCnt = A * A;
		else
  			iCnt = A * ( A + 1 ) + 1;

		cout << iCnt - 1 << endl;

		return 0;
	}

	if( R % 2 == 0 )
		iCnt = ( R / 2 ) * C;
	else if( C % 2 == 0 )
		iCnt = ( C / 2 ) * R;
	else
		iCnt = ( R / 2 ) * C + C / 2 + 1;

	cout << iCnt - 1 << endl;

	return 0;
}