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 Java8 (OpenJDK 1.8.0) (judge verdict: Runtime Error)

import java.util.Scanner;

public class Main {
    public static void main(String... args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        long[] a = new long[k + 1];

        for (int i = 1; i <= k; i++) {
            if (i == 1) {
                a[i] = nCr(n-k+1, i)%1000000007;
            } else {
                a[i] = (nCr(n-k+1, i)%1000000007) * (nCr(k-1,i-1)%1000000007);
                a[i] %= 1000000007;
            }
            System.out.println(a[i]);
        }

    }

    public static long nCr(int n, int r) {
        long num = 1;
        for (int i = 1; i <= r; i++) {
            num = num * (n - i + 1) / i;
            num %= 1000000007;
        }
        return num;
    }
}