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: Runtime Error)

def comb(n, r):
  if n < 0 or r < 0:
    return 0
  r = min(r, n - r)
  if n == 0 or r == 0:
    return 1
  return (n - r + 1) * comb(n, r - 1) // r

N, K = list(map(int, input().split()))
for i in range(K):
  print(comb(N - K + 1, i + 1) * comb(K - 1, i) % (10 ** 9 + 7))