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 C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;

int main() {
  string S;
  cin >> S;
  if (S.at(0)==S.at(1) && S.at(2)==S.at(3) && S.at(1)!=S.at(2)) {
    cout << "Yes" << endl;
  }
  else {
    if (S.at(0)==S.at(2) && S.at(1)==S.at(3) && S.at(1)!=S.at(2)) {
      cout << "Yes" << endl;
    }
    else {
      if (S.at(0)==S.at(3) && S.at(1)==S.at(2) && S.at(0)!=S.at(1)) {
        cout << "Yes" << endl;
      }
      else {
        cout << "No" << endl;
      }
    }
  }
}