#include "bits/stdc++.h"
using namespace std;
#define ll long long int
#define rep(i,n) for( int i = 0; i < n; i++ )
#define rrep(i,n) for( int i = n; i >= 0; i-- )
#define REP(i,s,t) for( int i = s; i <= t; i++ )
#define RREP(i,s,t) for( int i = s; i >= t; i-- )
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define INF 2000000000
#define mod 1000000007
#define INF2 1000000000000000000
#define int long long

signed main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
	int R, C; cin >> R >> C;
	if (R != C) {
		cout << (R * C + 1) / 2 - 1 << endl;
	}
	else {
		if (R % 2 == 0) {
			cout << R * R / 4 - 1 << endl;
		}
		else {
			cout << (R * R) / 4 << endl;
		}
	}

    return 0;
}