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 ll;
#define REP(i, n) for(ll i=0; i<(ll)n; i++)
#define REPR(i, n) for(ll i=(ll)n-1; i>=0; i--)
#define FOR(i, m, n) for(ll i=(ll)m; i<(ll)n; i++)
#define FORR(i, m, n) for(ll i=(ll)m-1; i>=(ll)n; i--)
#define ALL(x) (x).begin(), (x).end()
#define ASC(v) sort(ALL(v));
#define DESC(v) sort(ALL(v), greater<int>());
#define UNIQ(a) sort(ALL(a));(a).resize(unique(ALL(a)) - a.begin());
#define PRINTD(x, d) cout << fixed << setprecision(d) << x << "\n";
#define DEBUG(x) cout<<#x<<": "<<x<<"\n"
const double PI = acos(-1);
ll lcm(ll a, ll b) { return a / __gcd(a, b) * b; }
bool isprime(int x){ int i; if(x<2)return 0; else if(x==2) return 1; if(x%2==0) return 0; for(i=3; i*i<=x; i+=2) if(x%i==0) return 0; return 1; }
int digsum(int n) { int r=0; while(n>0) { r+=n%10; n/=10; } return r; }
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }

const string YES = "Yes";
const string NO = "No";

void solve(std::string S){
	bool can = false;
	map<char, ll> M;
	REP(i, S.size()) {
		M[S[i]] = 1;
	}
	if (M.size() == 2) can = true;
	if (can) cout << YES << "\n";
	else cout << NO << "\n";
}

int main(){
    std::string S;
    std::cin >> S;
    solve(S);
    return 0;
}