#include <bits/stdc++.h>
using namespace std;
const int maxint = -1u >> 1;
const int maxn = 100;
const int maxm = 100;
const int mmax = maxn * maxm + maxn + maxm + 100;
template <class T>
bool get_max(T &a, const T &b) {
  return b > a ? a = b, 1 : 0;
}
template <class T>
bool get_min(T &a, const T &b) {
  return b < a ? a = b, 1 : 0;
}
struct Graph {
  int n, m;
  int adj[maxn][maxm];
  int sz[maxm];
  void init() {
    n = m = 0;
    memset(adj, 0, sizeof(adj));
  }
  void insert(int u, int v) {
    get_max(n, u + 1);
    get_max(m, v + 1);
    adj[u][v] = 1;
  }
  int find_ans() {
    build_dlx();
    for (int ans = 0; ans <= n; ++ans) {
      if (dfs(0, ans)) {
        return ans;
      }
    }
    return -1;
  }
  int head;
  int U[mmax], D[mmax], L[mmax], R[mmax];
  int CN[mmax], RN[mmax];
  int rowhead[maxn];
  void addUD(const int a, const int x) {
    D[a] = x;
    U[a] = U[x];
    D[U[x]] = a;
    U[x] = a;
    CN[a] = x;
  }
  void addLR(const int a, const int x) {
    R[a] = x;
    L[a] = L[x];
    R[L[x]] = a;
    L[x] = a;
    RN[a] = x;
  }
  void add(const int &k, const int &r, const int &c) {
    addUD(k, c);
    addLR(k, r);
  }
  void remove(int &c) {
    for (int i = D[c]; i != c; i = D[i]) {
      L[R[i]] = L[i];
      R[L[i]] = R[i];
    }
  }
  void resume(int &c) {
    for (int i = U[c]; i != c; i = U[i]) {
      L[R[i]] = i;
      R[L[i]] = i;
    }
  }
  void build_dlx() {
    head = mmax - 1;
    U[head] = D[head] = L[head] = R[head] = head;
    int cnt = 0;
    for (int i = 0; i < m; ++i) {
      U[cnt] = D[cnt] = cnt;
      addLR(cnt++, head);
    }
    for (int i = 0; i < n; ++i) {
      rowhead[i] = -1;
      for (int j = 0; j < m; ++j) {
        if (adj[i][j]) {
          if (rowhead[i] == -1) {
            L[cnt] = R[cnt] = rowhead[i] = cnt;
          } else
            addLR(cnt, rowhead[i]);
          addUD(cnt, j);
          cnt++;
          sz[j]++;
        }
      }
    }
  }
  int h() {
    int hash[maxm] = {};
    int ans = 0;
    for (int c = R[head]; c != head; c = R[c]) {
      if (hash[c] == 0) {
        hash[c] = 1;
        ++ans;
        for (int j = D[c]; j != c; j = D[j]) {
          for (int i = R[j]; i != j; i = R[i]) {
            if (CN[i] != head) {
              hash[CN[i]] = 1;
            }
          }
        }
      }
    }
    return ans;
  }
  bool dfs(int deep, int lim) {
    if (deep + h() > lim) {
      return false;
    }
    if (R[head] == head) {
      return true;
    }
    int c = R[head];
    for (int j = R[head]; j != head; j = R[j])
      if (sz[j] < sz[c]) {
        c = j;
      }
    for (int i = D[c]; i != c; i = D[i]) {
      remove(i);
      for (int j = R[i]; j != i; j = R[j]) {
        remove(j);
      }
      if (dfs(deep + 1, lim)) {
        return true;
      }
      for (int j = L[i]; j != i; j = L[j]) {
        resume(j);
      }
      resume(i);
    }
    return false;
  }
} G;
int n, m;
bool inboard(int x, int y) { return 0 <= x && x < n && 0 <= y && y < m; }
int get_id(int x, int y) { return x * m + y; }
int dx[5] = {0, 0, 0, -1, 1};
int dy[5] = {0, -1, 1, 0, 0};
int main() {
  scanf("%d%d", &n, &m);
  int ans = n * m;
  for (int x = 0; x < n; x++) {
    for (int y = 0; y < m; y++) {
      for (int dir = 0; dir <= 4; dir++) {
        int tx = x + dx[dir], ty = y + dy[dir];
        if (inboard(tx, ty)) {
          G.insert(get_id(x, y), get_id(tx, ty));
        }
      }
    }
  }
  printf("%d\n", ans - G.find_ans());
  return 0;
}
