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: Runtime Error)

using System;
using static System.Console;

public class Hello
{
    const long p = 1000000007;
  	static string l = ReadLine();
    static readonly long N = long.Parse(l.Split()[0]);
    static readonly long K = long.Parse(l.Split()[1]);      
  
    public static void Main()
    {
       	
      for (long i=0;i<K;i++){
      WriteLine(((C(N-K+1,i+1) % p) * C(K-1,i)) % p);      
      }

    }
  
  	public static long C(long n,long m){
    if (m == 0) return 1;
    if (n == 0) return 0;
    return (n * C(n - 1, m - 1)/ m)% p;    
    }
}