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 (
	"bufio"
	"fmt"
	"os"
)

func main() {
	nextReader = newScanner()
	s := nextString()

	m := make(map[string]int)
	for _, c := range s {
		m[string(c)]++
	}
	if len(m) != 2 {
		fmt.Println("No")
		return
	}
	if m[string(s[0])] != 2 {
		fmt.Println("No")
		return
	}
	fmt.Println("Yes")
}

// ---------------------------------------------------------------
// I/O
// ---------------------------------------------------------------
var nextReader func() string

func newScanner() func() string {
	sc := bufio.NewScanner(os.Stdin)
	sc.Buffer(make([]byte, 1024), int(1e11))
	sc.Split(bufio.ScanWords)
	return func() string {
		sc.Scan()
		return sc.Text()
	}
}

func nextString() string { return nextReader() }