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;
typedef long long int ll;
#define rep( i, n, m ) for(int i = ( n ); i < ( m ); i++)
#define rep_d( i, n, m ) for(int i = ( n ) - 1; i >= ( m ); i--)
#define sort_asc( X ) sort(( X ).begin(), ( X ).end())
#define sort_desc( X ) sort(( X ).begin(), ( X ).end(), greater <>())

template <class T> bool chmax( T& a, T b );
template <class T> bool chmin( T& a, T b );
int* eratosthenes( int N );
bool bit_search( int pattern, int N );
template <class T> void cumulative_sum( T array, size_t N );

const ll INF = 1LL << 60;
const int k_mod = 1e9 + 7;

int main( void ){
	string str;
	cin >> str;
	map<char, int> mp;
	rep( i, 0, 4 ){
		mp[str[i]]++;
	}
	for(auto m:mp){
		if (m.second != 2){
			cout << "No\n";
			return 0;
		}
	}
	cout << "Yes\n";
}

int* eratosthenes( int N ){
	int* prime_array = new int[ N + 1 ];
	int lim = N;
	prime_array[ 0 ] = 0;
	prime_array[ 1 ] = 0;
	rep( i, 2, N + 1 ){
		prime_array[ i ] = 1;
	}
	rep( i, 2, lim ){
		if (prime_array[ i ] == 0)
			continue;
		lim = N / i;
		for(int j = i * 2; j < N + 1; j += i){
			prime_array[ j ] = 0;
		}
	}
	return prime_array;
}
template <class T> void cumulative_sum( T array, size_t N ) {
	rep( i, 1, N + 1 ){
		array[ i ] += array[ i - 1 ];
	}
}
template <class T> bool chmax( T& a, T b ) {
	if (a < b){
		a = b; return true;
	}
	return false;
}
template <class T> bool chmin( T& a, T b ) {
	if (a > b){
		a = b; return true;
	}
	return false;
}
bool bit_search( int pattern, int N ){
	int cnt = 0;
	rep( bit, 0, N ){
		if (pattern & ( 1 << bit )){
			cnt++;
		}
	}
	return true;
}