import std.algorithm, std.array, std.container, std.range;
import std.string, std.conv, std.bigint, std.math, std.random;
import std.stdio, std.typecons;

void main()
{
  auto rd = readln.split.map!(to!long);
  auto r = rd[0];
  auto c = rd[1];

  if (r == c) {
    if (r % 2 == 1)
      writeln((r * r - 1) / 4);
    else
      writeln(r * r / 4 - 1);
  } else {
    if (r % 2 == 1 && c % 2 == 1)
      writeln((r * c - 1) / 2);
    else
      writeln(r * c / 2 - 1);
  }
}