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 Kotlin (1.0.0) (judge verdict: Accepted)

import java.util.*
import java.math.*

fun main(args: Array<String>) {
//    var (N,K) = readLine()!!.split(" ").map { it.toInt() }
//    var N = readLine()!!.toInt()
    val S = readLine()!!
//    val D = readLine()!!.split(" ").map { it.toLong() }
    var ans = "Yes"
    var a:Char?=null
    var b:Char?=null
    var isA = false
    var isB = false
    for (c in S) {
        if (a==null) a=c
        else if (a==c) isA = isA xor true
        else if (b==null) b=c
        else if (b==c) isB = isB xor true
    }
    println(if (isA && isB) "Yes" else "No")
}