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 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
int main (){
  char s[5];
  int k=0;
scanf ("%s",s);
if (s[0]==s[1] && s[2]==s[3]){
k++;
}
  else if (s[0]==s[2] && s[1]==s[3]){
  k++;
  }
  else if (s[0]==s[3] && s[1]==s[2]){
  k++;
  }
  if (s[0]==s[1] && s[0]==s[2] && s[0]==s[3]){
  k=0;
  }
  if (k>0){
  printf ("Yes");
  }
 else {
 printf ("No");
 }
return 0;
}