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 JavaScript (Node.js 12.16.1) (judge verdict: Wrong Answer)

const fs = require('fs')

const input = fs.readFileSync('/dev/stdin', 'utf8')

const S = input.trim()

const counter = {}

for (let i=0; i<4; i++) {
  const c = S.substr(i, 1)
  if (c in counter) {
    counter[c] = counter[c] + 1
  } else {
    counter[c] = 1    
  }
}

if (counter[S.substr(0, 1)] === 2) {
  console.log('Yes')
} else {
  console.log('No')
}