let [| h; w |] = stdin.ReadLine().Split(" ") |> Array.map bigint.Parse
let two = bigint.Parse "2"
let ans = 
  if h % two = bigint.Zero then 
    w * h / two
  else 
    w * (h - bigint.One) / two + (w / two) + bigint.One   
ans |> string |> printfn "%s"