import std.stdio, std.conv, std.math, std.string, std.range, std.array,
       std.algorithm;

void main(){
  auto buf = readln().strip().split().map!(to!long)();
  immutable R = buf[0];
  immutable C = buf[1];

  long ans = R * C;
  bool odd = false;
  if(R%2 == 1 && C%2 == 1) {
    --ans;
    odd = true;
  }

  int divn = 2;
  if (R == C) divn *= 2;

  writeln(ans/divn - 1 + (odd?1:0));
}