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: Wrong Answer)

#include<stdio.h>
int main()
{
	int k=0;
	char s[5],a1,a2;
	gets(s);
	a1=s[0];
	for(int i=1;i<4;i++)
	{
		if(s[i]!=a1)
		{
			a2=s[i];
			k=i+1;
			break;
		}
	}
	for(int i=k;i<4;i++)
	{
		if(s[i]!=a1&&s[i]!=a2)
		{
			printf("No");
			return 0;
		}
	}
	printf("Yes");
}