(input) =>
 
let
 
values = {
 
  {1, "Jul"},
  {2, "Aug"},
  {3, "Sep"},
  {4, "Oct"},
  {5, "Nov"},
  {6, "Dec"},
  {7, "Jan"},
  {8, "Feb"},
  {9, "Mar"},
  {10, "Apr"},
  {11, "May"},
  {12, "Jun"},
  {input, "Undefined"}
 
},
 
Result = List.First(List.Select(values, each _{0}=input)){1}
 
in
 
Result