let rec safe x y posl =
  match posl with
    (x1,y1) :: xs -> 
           x <> x1          (* not same column   *)
        && y <> y1          (* not same row      *)
        && y - x <> y1 - x1 (* not on /-diagonal *)
        && y + x <> y1 + x1 (* not on \-diagonal *)
        && safe x y xs
  | _ -> true

          
exception Conflict

let queens4 n =
    let rec queens x y posl =
      if x > n then posl                 (*   1 *)
      else if y > n then raise Conflict  (* 2.b *)
      else try if safe x y posl              (* 2.a *) 
               then queens (x+1) 1 ((x,y) :: posl) 
               else raise Conflict
           with Conflict -> queens x (y+1) posl
    in  queens 1 1 []


let doit = 
  let sol = queens4 (int_of_string(Sys.argv.(1))) in
  let printPair (x, y) = (print_int x;
                          print_string ", ";
                          print_int y;
                          print_newline())
  in  List.iter printPair sol