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;
use std::collections::HashMap;

fn main() {
	let mut buf = String::new();
	io::stdin().read_line(&mut buf).expect("");
	let s:Vec<char> = buf.trim().chars().collect();
	let mut m = HashMap::new();
	for i in s {
		match m.insert(i, 1) {
			None => (),
			Some(n) => {
				m.insert(i, n+1);
			},
		}
	}
	let mut flag = true;
	for val in m.values() {
		if *val != 2 {
			flag = false;
			break;
		}
	}
	if flag {
		println!("Yes");
	}else{
		println!("No");
	}
}