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 static System.Console;

class Program
{
    public static void Main()
    {
        string s = ReadLine();
        int ct1 = 0, ct2 = 0;
        char c1 = '\0', c2 = '\0';

        for (int i = 0; i < 4; i++)
        {
            char c = s[i];

            if      (c == c1){ ct1++; }
            else if (c == c2){ ct2++; }
            else if (c1 == '\0')
            {
                c1 = c;
                ct1++;
            }
            else if (c2 == '\0')
            {
                c2 = c;
                ct2++;
            }
        }

        bool good = (ct1 == 2) && (ct2 == 2);
        WriteLine(good?"Yes":"No");
    }
}