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++ (Clang 3.8.0) (judge verdict: Accepted)

#include <iostream>
#include <iomanip>
#include <stdlib.h>
#include <cmath>
#include <string>
#include <cstring>
#include <utility>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#define debug(x) cerr << #x << ':' << x << endl
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define ALL(c) (c).begin(), (c).end()
using namespace std;
typedef long long ll;

int main(){
    string s;
    cin >> s;
    set<char> se;
    for(auto c : s){
        se.insert(c);
    }
    
    if(se.size()==2){
        int cnt=0;
        for(auto c : s){
            if(s[0] == c) cnt++;
        }
        if(cnt==2) cout << "Yes" << endl;
        else cout << "No" << endl;
            }
    else{
        cout << "No" << endl;
    }
    
    return 0;
}