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 <iostream>
#include <string>
#include <algorithm>
#include <cassert>
#include <limits>
#include <numeric>
#include <type_traits>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <functional>
#include <utility>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

typedef long long ll;
#define FOR(i,a,b) for(ll i=(a);i<(b);i++)
#define REP(i,n)  FOR(i,0,n)

int main(){
	string S;
	cin >> S;
	string S1 = S.substr(0,1);
	string S2 = S.substr(1,1);
	string S3 = S.substr(2,1);
	string S4 = S.substr(3,1);
	if(S1 == S2 && S3 == S4){
		if(S2 != S3){
		cout << "Yes" << endl;
	}
	else{
		cout << "No" << endl;
		}
	}
	else if(S2 == S3 && S1 == S4){
		if(S2 != S1){
			cout << "Yes" << endl;
		}
		else{
			cout << "No" << endl;
		}
	}
	else if(S1 == S3 && S2 == S4){
		if(S1 != S2){
			cout << "Yes" << endl;
		}
		else{
			cout << "No" << endl;
		}
	}
	else{
		cout << "No" << endl;
	}
}