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 v5.12) (judge verdict: Wrong Answer)

"use strict"

const main = (input) => {
    input = input.split("");
    const S = new Set(input);
    let sum1 = 0;
    let sum2 = 0;

    for (let i = 0; i < input.length; i++){
        if (input[0] == input[i]) {
            sum1 += 1;
        } else {
            sum2 += 1;
        }
    }

    if (S.size == 2 && sum1 == 2 && sum2 == 2) {
        console.log("Yes");
    } else {
        console.log("No");
    }
};

main(require("fs").readFileSync("/dev/stdin", "utf8"));