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: Wrong Answer)

import sys
a1=sys.stdin.readline()
a2=list(a1)
b=[ord(x) for x in a2]
b.sort()
if b[0]==b[1] and b[2]==b[3] and b[0]!=b[3] and b[1]!=b[2] and len(a2)==4:
    print("Yes")
elif len(a2)==4:
    print("No")
else:
    print("Error")