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 (4.10.0) (judge verdict: Accepted)

let fifty s =
  if s.[0] = s.[1] && s.[2] = s.[3] && s.[0] <> s.[2] then "Yes"
  else if s.[0] = s.[2] && s.[1] = s.[3] && s.[0] <> s.[1] then "Yes"
  else if s.[0] = s.[3] && s.[1] = s.[2] && s.[0] <> s.[1] then "Yes"
  else "No"

let () = Printf.printf "%s\n" (fifty (read_line ()))