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 (2.7.6) (judge verdict: Runtime Error)

facts = [1]
mod =  10**9 + 7
for i in range(1, 2010):
	facts.append((i*facts[-1])%mod)
modinv = []
for i in facts:
	modinv.append(pow(i, mod - 2, mod))

def choose(p, r):
	return (((facts[p]*modinv[r])%mod)*modinv[p-r])%mod

n, k = map(int, raw_input().split())
blue = k
red = n - k
for i in range(1, k+1):
	if i <= n - k + 1:
		print (choose(red + 1, i)*choose(k - 1, i - 1))%mod
	else:
		print 0