#include <bits/stdc++.h>
int dom(int x, int y) {
  int M, Ans;
  if (x % 2 == 0) {
    M = x / 2;
    Ans = y * M;
    return Ans;
  }
  M = x / 2;
  Ans = y * M + y / 2;
  return Ans;
}
int main() {
  int M, N, Ans;
  scanf("%d%d", &M, &N);
  Ans = dom(M, N);
  printf("%d", Ans);
}
