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 Python (3.4.3) (judge verdict: Accepted)

def main():
    S = input()
    count = {}
    for s in S:
        if not s in count.keys():
            count[s] = 0
        count[s] += 1
    count_sorted = sorted(count.items(), key=lambda x:-x[1])
    if count_sorted[0][1] == 2 and count_sorted[1][1] == 2:
        print("Yes")
    else:
        print("No")
    return


if __name__ == "__main__":
    main()