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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
int a[1000];
int main()
{
    char s[10];
    scanf("%s",s+1);
    for(int i=1;i<=4;i++)
        a[s[i]]++;
    int flag=0;
    for(int i=1;i<=4;i++)
    {
        if(a[s[i]]!=2)
            flag=1;
    }
    if(flag==1)
        printf("No\n");
    else printf("Yes\n");
    return 0;
}