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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;
import java.math.*;
class Main{
public static void main(String[] args) {
  Scanner stdIn = new Scanner(System.in);
  String s = stdIn.next();
  char[] a = new char[s.length()];
  int counta = 0;
  int countb = 0;
  char b = ' ';
  char c = ' ';
  int k = 0;
  for(int i = 0;i<a.length;i++) {
    a[i] = s.charAt(i);
  }
  b = a[0];
  for(int i = 0;i<a.length;i++) {
    if(b==a[i]) counta += 1;
    else if(k==0){
      k += 1;
      c = a[i];
      countb += 1;
    } else if(k!=0&&c==a[i]) {
      countb += 1;
    }
  }
  if(counta==countb&&counta==2&&countb==2) {
    System.out.println("Yes");
  } else {
    System.out.println("No");
  }

}
}