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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ABC132A
{
    class Program
    {
        static void Main(string[] args)
        {
            string S = Console.ReadLine();
            char[] A = new char[4];
            A[0] = S[0];
            A[1] = S[1];
            A[2] = S[2];
            A[3] = S[3];
            string ans = "No";
            if (A[0]==A[1]&&A[2]==A[3]&&A[0]!=A[2])
            {
                ans = "Yes";
            }
            if (A[0]==A[2]&&A[1]==A[3]&&A[0]!=A[1])
            {
                ans = "Yes";
            }
            if (A[0]==A[3]&&A[1]==A[2]&&A[0]!=A[1])
            {
                ans = "Yes";
            }
            Console.WriteLine(ans);
            //入力例
            // ASSA
            // STOP
            // FFEE
            // FREE
        }
    }
}