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;
typedef unsigned long long ull;
using P = pair<ll, ll>;
#define rep(i, n) for(ll i=0;i<(ll)(n);i++)
#define rep2(i, m, n) for(ll i=m;i<(ll)(n);i++)
#define rrep(i, n, m) for(ll i=n;i>=(ll)(m);i--)
#define ALL(obj) (obj).begin(), (obj).end()
const int INF = 1e9;

int main(void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	string S;
	string ans;

	cin >> S;

	vector<char> c;
	c.push_back(S[0]);
	rep2(i, 1, S.size()) {
		auto itr = find(c.begin(), c.end(), S[i]);
		if (itr == c.end()) {
			c.push_back(S[i]);
		}
	}

	if (c.size() != 2) ans = "No";
	else ans = "Yes";
	cout << ans << "\n";

	return 0;
}