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 Rust (1.15.1) (judge verdict: Accepted)

use std::io::Read;
use std::cmp;

fn main() {
    let mut s = String::new();

    std::io::stdin().read_line(&mut s).unwrap();

    let vecs: Vec<char> = s.chars().collect();
    let cnt_1st_char: usize = s.match_indices(vecs[0]).count();
    let cnt_2nd_char: usize = s.match_indices(vecs[1]).count();
    let cnt_3rd_char: usize = s.match_indices(vecs[2]).count();
    let cnt_4th_char: usize = s.match_indices(vecs[3]).count();

    if cnt_1st_char == 2 && cnt_2nd_char == 2 && cnt_3rd_char == 2 && cnt_4th_char == 2 {
        println!("{}", "Yes");
    } else {
        println!("{}", "No");
    }

}