#include "bits/stdc++.h"
using namespace std;
//諸機能
#pragma region MACRO
#define ANSWER(x) cout << (x) << endl
#define DOUBLE_ANSWER(x) cout << set_presicion(10) << (x) << endl
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define ALL(a) begin((a)),end((a))
#define FILL(a,n) for(auto &hoge : (a)) hoge = (n)
#define FILL_NDIM_ARRAY(arr,size,val) fill(*(arr),(*(arr) + (size)),(val))
#define mp make_pair
#define EXIST(container, n) ((container).find((n)) != (container).end())
#define STOI(s,i,l) stoi(SUBSTR(s,i,l))
#define SUBSTR(s,i,l) string((s), (i), (l))
#define NPI_TO_RAD(x) (180.0*x/PI)
#define RAD_TO_NPI(x) (PI/180.0*x)
#pragma endregion
//デバッグなどの支援
#pragma region CODING_SUPPORT
#define DEBUG1(var0) { std::cerr << (#var0) << "=" << (var0) << endl; }
#define DEBUG2(var0, var1) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG1(var1); }
#define DEBUG3(var0, var1, var2) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG2(var1,var2); }
#define DEBUG4(var0, var1, var2, var3) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG3(var1,var2,var3); }
#pragma endregion
//typedef（書き換える、書き足す可能性ある）
#pragma region TYPE_DEF
typedef long long ll; 
typedef pair<int,int> pii;
typedef pair<string,string> pss;
typedef pair<int,string>pis;
typedef pair<string,int>psi;
typedef vector<string> vs;
typedef vector<int> vi;
#pragma endregion
//諸々の定数(書き換える可能性ある)
#pragma region CONST_VAL
#define PI (2*acos(0.0))
#define EPS (1e-9)
#define MOD (1e9 + 7)
#pragma endregion

int main()
{
    ll r,c;
    ll ans = 0;
    cin >> r >> c;
    if (r % 2 == 0 )
    {
        ans += (r/2)*c - 1;
    }
    else
    {
        ans += (r/2)*c;
        ans += c/2;
    }
    ANSWER(ans);
    return 0;
    
}