let ``No.799 赤黒かーどげぇむ``() =

  let a, b, c, d = 
    stdin.ReadLine().Split(' ')
    |> Array.map (int)
    |> fun args -> args.[0], args.[1], args.[2], args.[3]

  seq { 
    for i in [a..b] do  
      for j in [c..d] do
        if i <> j 
          then yield (i,j)
     }
  |> fun sq -> 
    stdout.WriteLine (sq |> Seq.length)
  
  ()
``No.799 赤黒かーどげぇむ``()