Scanf.sscanf (read_line ()) "%d %d %d" (fun k n f ->
               let line2 = read_line () in
               let nums = List.map (fun s -> int_of_string s)
                                   (Str.split (Str.regexp " ") line2) in
               let sum = List.fold_left (+) 0 nums in
               let diff = (k * n) - sum
               in
               if diff < 0 then
                 print_int (-1)
               else
                 print_int diff;
               print_newline ());;