Problem p04013 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

#define CPP_STR(x) CPP_STR_I(x)
#define CPP_CAT(x, y) CPP_CAT_I(x, y)
#define CPP_STR_I(args...) #args
#define CPP_CAT_I(x, y) x##y

using i8 = int8_t;
using u8 = uint8_t;
using i16 = int16_t;
using u16 = uint16_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;

using f32 = float;
using f64 = double;

#define FOR(i, start, end)                                                     \
  for (i64 i = (start), CPP_CAT(i, xxxx_end) = (end);                          \
       i < CPP_CAT(i, xxxx_end); ++i)
#define REP(i, n) FOR(i, 0, n)
#define REPS(i, n) for (i64 i = 1; i <= n; ++i)
#define RREP(i, n) for (i64 i = n - 1; i >= 0; --i)
#define RREPS(i, n) for (i64 i = n; i >= 1; --i)

#define ALL(f, c, ...)                                                         \
  (([&](decltype((c)) cccc) {                                                  \
    return (f)(std::begin(cccc), std::end(cccc), ##__VA_ARGS__);               \
  })(c))

#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

#define pb push_back
#define mp make_pair

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

template <typename C> i64 SIZE(const C &c) {
  return static_cast<i64>(c.size());
}

template <typename T, size_t N> i64 SIZE(const T (&)[N]) {
  return static_cast<i64>(N);
}

struct ProconInit {
  static constexpr int IOS_PREC = 15;
  static constexpr bool AUTOFLUSH = false;

  ProconInit() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(IOS_PREC);
    if (AUTOFLUSH)
      cout << unitbuf;
  }
} PROCON_INIT;

#define MAX_N 50
#define MAX_A 50
#define MAX_S 2500
i64 dp[MAX_N + 1][MAX_N + 1][MAX_S + 1];

int main() {
  i64 N, A;
  cin >> N >> A;
  vector<i64> x(N);
  REP(i, N) cin >> x[i];

  dp[0][0][0] = 1;
  REP(i, N) {
    REP(j, N + 1) {
      REP(k, MAX_S + 1) {
        dp[i + 1][j][k] += dp[i][j][k];
        if (k + x[i] <= MAX_S) {
          dp[i + 1][j + 1][k + x[i]] += dp[i][j][k];
        }
      }
    }
  }
  i64 cnt = 0;
  REPS(i, N) { cnt += dp[N][i][i * A]; }
  cout << cnt << endl;

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

#define CPP_STR(x) CPP_STR_I(x)
#define CPP_CAT(x, y) CPP_CAT_I(x, y)
#define CPP_STR_I(args...) #args
#define CPP_CAT_I(x, y) x##y

using i8 = int8_t;
using u8 = uint8_t;
using i16 = int16_t;
using u16 = uint16_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;

using f32 = float;
using f64 = double;

#define FOR(i, start, end)                                                     \
  for (i64 i = (start), CPP_CAT(i, xxxx_end) = (end);                          \
       i < CPP_CAT(i, xxxx_end); ++i)
#define REP(i, n) FOR(i, 0, n)
#define REPS(i, n) for (i64 i = 1; i <= n; ++i)
#define RREP(i, n) for (i64 i = n - 1; i >= 0; --i)
#define RREPS(i, n) for (i64 i = n; i >= 1; --i)

#define ALL(f, c, ...)                                                         \
  (([&](decltype((c)) cccc) {                                                  \
    return (f)(std::begin(cccc), std::end(cccc), ##__VA_ARGS__);               \
  })(c))

#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

#define pb push_back
#define mp make_pair

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

template <typename C> i64 SIZE(const C &c) {
  return static_cast<i64>(c.size());
}

template <typename T, size_t N> i64 SIZE(const T (&)[N]) {
  return static_cast<i64>(N);
}

struct ProconInit {
  static constexpr int IOS_PREC = 15;
  static constexpr bool AUTOFLUSH = false;

  ProconInit() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(IOS_PREC);
    if (AUTOFLUSH)
      cout << unitbuf;
  }
} PROCON_INIT;

#define MAX_N 50
#define MAX_A 50
#define MAX_S 2500
i64 dp[MAX_N + 1][MAX_N + 1][MAX_S + 1];

int main() {
  i64 N, A;
  cin >> N >> A;
  vector<i64> x(N);
  REP(i, N) cin >> x[i];

  dp[0][0][0] = 1;
  REP(i, N) {
    for (i64 j = 0; j <= i; j++) {
      REP(k, MAX_S + 1) {
        dp[i + 1][j][k] += dp[i][j][k];
        if (k + x[i] <= MAX_S) {
          dp[i + 1][j + 1][k + x[i]] += dp[i][j][k];
        }
      }
    }
  }
  i64 cnt = 0;
  REPS(i, N) { cnt += dp[N][i][i * A]; }
  cout << cnt << endl;

  return 0;
}