Problem:
Score : 100 points

Problem Statement

You are given a 4-character string S consisting of uppercase English letters.
Determine if S consists of exactly two kinds of characters which both appear twice in S.

Constraints

The length of S is 4.

S consists of uppercase English letters.

Input

Input is given from Standard Input in the following format:

S

Output

If S consists of exactly two kinds of characters which both appear twice in S, print Yes; otherwise, print No.

Sample Input 1

ASSA

Sample Output 1

Yes

S consists of A and S which both appear twice in S.

Sample Input 2

STOP

Sample Output 2

No

Sample Input 3

FFEE

Sample Output 3

Yes

Sample Input 4

FREE

Sample Output 4

No

Solution in OCaml (4.02.3) (judge verdict: Accepted)

open Printf
open Scanf

let solve s =
  let l = List.sort compare @@ List.map (fun i -> String.get s i) [0;1;2;3] in
  match l with
    a::b::c::d::_ when a = b && c = d && b <> c -> "Yes"
  | _ -> "No"

let () =
  scanf "%s " solve |> printf "%s\n"