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 System.Linq;

class Program
{
    static void Main(string[] args)
    {
        int[] line = Console.ReadLine().Split().Select(int.Parse).ToArray();

        int ans = 0;
        for (int i = 1; i < line.Length - 1; i++)
        {
            var target = line[i];
            var one = line[i - 1];
            var two = line[i + 1];

            if (one < target && target < two)
            {
                ans++;

            }
            else if (two < target && target < one)
            {
                ans++;
            }
        }

        Console.WriteLine(ans);
    }
}