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: Wrong Answer)

use std::io;

fn main() {
    let mut raw_s: String = String::new();
    io::stdin().read_line(&mut raw_s).ok();
    let c_vec: Vec<char> = raw_s.as_str().chars().collect();
    let mut obj1: char = c_vec[0];
    let mut obj2: char = '\0';
    let mut num: i16 = 0;
    for c in raw_s.as_str().chars(){
        if obj1 != c && obj2 == '\0' {
            obj2 = c;
        }

        if obj1 != '\0' && obj1 == c {
            num += 1;
            obj1 = '\0';
        }

        if obj2 != '\0' && obj2 != '\t' && obj2 == c {
            num += 1;
            obj2 = '\t';
        }
    }

    if num == 2 {
        println!("Yes");
    }else{
        println!("No");
    }
}