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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"fmt"
)

func main() {
	var s string
	fmt.Scan(&s)
	m := make(map[byte]int, 0)
	for i := 0; i < 4; i++ {
		m[s[i]]++
	}
	
	for i := 0; i < 4; i++ {
		if len(m) > 2 || m[s[i]] > 2 {
			fmt.Println("No")
			return
		}
	}
	fmt.Println("Yes")
}