Problem:
Score : 300 points

Problem Statement

Tak has N cards. On the i-th (1 \leq i \leq N) card is written an integer x_i.
He is selecting one or more cards from these N cards, so that the average of the integers written on the selected cards is exactly A.
In how many ways can he make his selection?

Constraints

1 \leq N \leq 50

1 \leq A \leq 50

1 \leq x_i \leq 50

N,\,A,\,x_i are integers.

Partial Score

200 points will be awarded for passing the test set satisfying 1 \leq N \leq 16.

Input

The input is given from Standard Input in the following format:

N A
x_1 x_2 ... x_N

Output

Print the number of ways to select cards such that the average of the written integers is exactly A.

Sample Input 1

4 8
7 9 8 9

Sample Output 1

5

The following are the 5 ways to select cards such that the average is 8:

Select the 3-rd card.

Select the 1-st and 2-nd cards.

Select the 1-st and 4-th cards.

Select the 1-st, 2-nd and 3-rd cards.

Select the 1-st, 3-rd and 4-th cards.

Sample Input 2

3 8
6 6 9

Sample Output 2

0

Sample Input 3

8 5
3 6 2 8 7 6 5 9

Sample Output 3

19

Sample Input 4

33 3
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

Sample Output 4

8589934591

The answer may not fit into a 32-bit integer.

Solution in C (GCC 9.2.1) (judge verdict: Accepted)

#include<stdio.h>
int main(){
  int n, a; scanf("%d %d", &n, &a);
  int v[n]; for(int i = 0; i < n; ++i) scanf("%d", &v[i]);
  long dp[n+1][n+1][50*n+1];
  for(int i = 0; i <= n; ++i) for(int j = 0; j <= n; ++j) for(int k = 0; k <= 50*n; ++k) dp[i][j][k] = 0;
  dp[0][0][0] = 1;
  for(int i = 0; i < n; ++i){
    for(int j = 0; j <= i; ++j){
      for(int k = 0; k <= 50*i; ++k){
        dp[i+1][j][k] += dp[i][j][k]; dp[i+1][j+1][k+v[i]] += dp[i][j][k];
      }
    }
  }
  long ans = 0;
  for(int i = 1; i <= n; ++i) ans += dp[n][i][a*i];
  printf("%ld\n", ans);
  return 0;
}