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::stdin;
use std::collections::HashMap;

fn main() {
    let mut s = String::new();
    stdin().read_line(&mut s).unwrap();
    let mut m: HashMap<char, usize> = HashMap::new();
    for a in s.trim().chars() {
        if !m.contains_key(&a) {
            m.insert(a, 1);
        } else if let Some(x) = m.get_mut(&a) {
            *x += 1
        }
    }
    if m.keys().count() == 2 && m.iter().all(|(_, &a)| a == 2) {
        println!("Yes");
    } else {
        println!("No");
    }
}