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)

# coding: utf-8
# Your code here!

n = input()
a1 = n[0]
a2 = n[0]
count1 = 1
count2 = 0
for nc in range(1,4):
    if(n[nc] == a1):
        count1 += 1
    elif(a1 == a2):
        a2 = n[nc]
        count2 += 1
    elif(n[nc] == a2):
        count2 += 1
    else:
        count1 = 5
if(count1 == count2):
    print("Yes")
else:
    print("No")