#define _CRT_SECURE_NO_WARNINGS
#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii;
typedef long long ll; typedef vector<long long> vl; typedef pair<long long,long long> pll; typedef vector<pair<long long,long long> > vpll;
typedef vector<string> vs; typedef long double ld;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

template<typename T, size_t WW>
void rotate_left_square(T a[][WW], int n) {
	int n1 = n / 2, n2 = n - n1;
	rep(j, n1) rep(i, n2) {
		//a[j][i], a[n-1-i][j], a[n-1-j][n-1-i], a[i][n-1-j]
		T t = a[i][n-1-j];
		a[i][n-1-j] = a[n-1-j][n-1-i];
		a[n-1-j][n-1-i] = a[n-1-i][j];
		a[n-1-i][j] = a[j][i];
		a[j][i] = t;
	}
}
template<typename T, size_t WW>
void rotate_left(T a[][WW], int H, int W) {
	rotate_left_square(a, max(H, W));
	int pad = max(0, H - W);
	if(pad > 0) {
		for(int i = 0; i < W; i ++)
			memcpy(a[i], a[i+pad], H * sizeof(**a));
	}
}
int main() {
	/*
	{	const int X = 8;
		bool a[X][X];
		rer(R, 1, X) rer(C, 1, X) {
			set<unsigned long long> s;
			rep(ty, R) rep(tx, C) {
				rep(y, R) rep(x, C)
					a[y][x] = true;
				a[ty][tx] = false;
				unsigned long long u = ~0ULL;
				rep(k, 4) {
					unsigned long long t = 0;
					rep(y, R) rep(x, C)
						t |= (unsigned long long)a[y][x] << (y*max(R,C)+x);
					amin(u, t);
					rotate_left(a, R, C);
					swap(R,C);
				}
				s.insert(u);
			}
			cerr << R << ", " << C << ": " << s.size() << endl;
		}

	}
	*/
	ll R, C;
	cin >> R >> C;
	ll ans;
	if(R != C) {
		ans = (R * C + 1) / 2;
	}else {
		ans = (R * R + 3) / 4;
	}
	cout << ans - 1 << endl;
	//ans = total - 90'
	return 0;
}