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;

namespace debug
{
    public static partial class Program
    {
        public static void Main()
        {
            var str = Console.ReadLine();
            var dist = str.Distinct();
            if (dist.Count() == 2)
            {
                foreach (var dis in dist)
                {
                    if (str.Count(x => x == dis) != 2)
                    {
                        Console.WriteLine("No");
                        return;
                    }
                }
            }
            else
            {
                Console.WriteLine("No");
                return;
            }
            
            Console.WriteLine("Yes");
        }
    }

    public static partial class Program
    {
        //テンプレここから
        public static long a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z;
        public static long GCD(long aaa, long bbb) => bbb == 0 ? aaa : GCD(bbb, aaa % bbb);
        public static long LCM(long aaa, long bbb) => aaa * bbb / GCD(aaa, bbb);
        public static long ToLong(this double ddd) => (long) ddd;
        public static long ToLong(this string str) => long.Parse(str);
        public static long[] ToLong(this IEnumerable<string> strs) => strs.Select(x => x.ToLong()).ToArray();
        public static long[] ArrayLongs(this string str, char opr = ' ') => str.Split(opr).ToLong();
        public static long Diff(long aaa, long bbb) => Math.Abs(Math.Max(aaa, bbb) - Math.Min(aaa, bbb));
        public static long Sign(this long iii) => Math.Sign(iii);
        public static bool IsEven(this long iii) => iii % 2 == 0;
        public static readonly int[] Dx = {0, -1, 0, 1};
        public static readonly int[] Dy = {-1, 0, 1, 0};
        public const int MOD = 1000000007;
        public const string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        public const string ALPHABET = "abcdefghijklmnopqrstuvwxyz";
        public static long ToLong(this char ccc) => ccc.ToString().ToLong();
        public static string Join(this IEnumerable<string> array, string sep = "") => string.Join(sep, array);
        public static string Join(this IEnumerable<char> array, string sep = "") => string.Join(sep, array);
        public static string JoinToStr(this IEnumerable<long> array, string sep = "")
            => array.Select(xxx => xxx.ToString()).Join(sep);
        //テンプレここまで
    }
}