let () =
    let px, py, qx, qy = Scanf.scanf "%d %d\n%d %d\n" (fun a b c d -> a,b,c,d) in
    let x, y = abs (px-qx), abs (py-qy) in
    let numb = if (x+y)/2*2 = x+y then true else false in 
    Printf.printf "%s\n" (if numb then string_of_int ((x+y)/2) else string_of_float ((float_of_int (x+y)) /. 2.))