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::collections::HashMap;
use std::io::{self, Read};

fn main() {
    let mut buffer = String::new();
    io::stdin().read_to_string(&mut buffer).unwrap();
    let mut dist = HashMap::new();
    for c in buffer.trim().chars() {
        let count = dist.entry(c).or_insert(0);
        *count += 1;
    }
    if dist.len() == 2 && dist.values().all(|x| *x == 2) {
        println!("Yes");
    } else {
        println!("No");
    }
}