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>
using namespace std;

int main() {
	string S;
	cin>>S;
	set<char> st;
	bool flag=true;
	for(int i=0;i<4;i++){
		st.insert(S[i]);
	}
	if(st.size()!=2) flag=false;
	int cnt=0;
	for(int i=1;i<4;i++){
		if(S[i]==S[0]) cnt++;
	}
	if(cnt!=1) flag=false;
	cout<<(flag? "Yes":"No");
	return 0;
}