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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
#include<iostream>
using namespace std;
int main(){
  string S;
  cin >> S;
  vector<string> k(4);
  for(int i = 0; i < 4; i++)
    k.at(i) = S[i];
  int check = 0;
  int p = 0;
  for(int i = 1; i < 4; i++){
    if(k.at(0) == k.at(i)){
      check++;
      p = i;
      break;
    }
  }
  int chk = 0;
  for(int i = 1; i < 3; i++){
    if(i == p)
      continue;
    for(int j = i; j < 4; j++){
      if(j == p || i == j)
        continue;
      else{
        if(k.at(i) == k.at(j) && k.at(i) != k.at(0) && k.at(j) != k.at(0))
          chk++;
      }
    }
  }
  if(check && chk)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}