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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
int main()
{
	char a[4], s = '\0', k = '\0';
	gets(a);
	s = a[0];
	for (int i = 1; i < 4; i++)
	{
		if (a[i] != s && k == '\0')
			k = a[i];
		else if (a[i] != s && a[i] != k)
		{
			printf("No");
			return 0;
		}

	}
	if (k == '\0')
	{
		printf("No");
	}

	else
	{
		printf("Yes");
	}

}