#include <bits/stdc++.h>
using namespace std;
int main() {
  int M, N, a, b, c;
  cin >> M >> N;
  if (M % 2 == 0) {
    a = M / 2;
    b = N;
    c = a * b;
  } else if (N % 2 == 0) {
    a = N / 2;
    b = M;
    c = a * b;
  } else {
    a = (M - 1) / 2;
    b = N;
    c = a * b + (N / 2);
  }
  cout << c;
}
