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 C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>

// 赤の玉n個をm個の線を入れる
// nCm
long long int func1(int n, int m){
  int i;
  long long int rslt = 1;
  if (m == n || m == 0) {
    return rslt;
  }
  if (n < 2*m) {
    m = n - m;
  }
  for (i=1; i<=m; i++) {
    rslt *= n-i+1;
  }
  for (i=1; i<=m; i++) {
    rslt /= i;
  }
  return rslt;
}

int main() {
  int n, k;
  scanf("%d", &n);
  scanf("%d", &k);
  int i;
  for (i=1; i<=k; i++) {
    long long int a = func1(n-k+1, i) % 1000000007;
    long long int b = func1(k-1, i-1) % 1000000007;
    printf("%lld\n", a*b%1000000007);
  }
  return 0;
}