Problem p02851 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

#define int long long
#define rep(i, a, b) for (int i = (a); i < (b); i++)
#define rrep(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define all(c) begin(c), end(c)
const int INF =
    sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = 1000000007;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

template <typename T> vector<T> gen_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto gen_v(size_t a, Ts... ts) {
  return vector<decltype(gen_v<T>(ts...))>(a, gen_v<T>(ts...));
}
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N, K;
  cin >> N >> K;
  vector<int> A(N);
  rep(i, 0, N) { cin >> A[i]; }

  vector<int> B = A;
  rep(i, 0, N - 1) {
    B[i] %= K;
    B[i + 1] += B[i];
  }
  B.back() %= K;

  dump(B);

  auto f = [&](int x) {
    if (x < 0) {
      int k = abs(x) / K;
      x += (k + 1) * K;
      x %= K;
    }
    x %= K;
    return x;
  };

  rep(i, 0, N) {
    B[i] = i + 1 - B[i];
    B[i] = f(B[i]);
  }
  dump(B);

  int ans = 0;
  map<int, int> mp;
  rep(i, 0, K - 1) { mp[B[i]]++; }

  int x = 0;
  rep(i, 0, N) {
    dump(mp);
    dump(x, mp[x]);
    ans += mp[x];
    dump(ans);
    x++;
    x -= A[i];
    x = f(x);
    dump(x);
    mp[B[i]]--;
    if (i + K - 1 < N)
      mp[B[i + K - 1]]++;
  }
  cout << ans << endl;

  return 0;
}

Fixed submission:
#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

#define int long long
#define rep(i, a, b) for (int i = (a); i < (b); i++)
#define rrep(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define all(c) begin(c), end(c)
const int INF =
    sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = 1000000007;
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

template <typename T> vector<T> gen_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto gen_v(size_t a, Ts... ts) {
  return vector<decltype(gen_v<T>(ts...))>(a, gen_v<T>(ts...));
}
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N, K;
  cin >> N >> K;
  vector<int> A(N);
  rep(i, 0, N) { cin >> A[i]; }

  vector<int> B = A;
  rep(i, 0, N - 1) {
    B[i] %= K;
    B[i + 1] += B[i];
  }
  B.back() %= K;

  dump(B);

  auto f = [&](int x) {
    if (x < 0) {
      int k = abs(x) / K;
      x += (k + 1) * K;
      x %= K;
    }
    x %= K;
    return x;
  };

  rep(i, 0, N) {
    B[i] = i + 1 - B[i];
    B[i] = f(B[i]);
  }
  dump(B);

  int ans = 0;
  map<int, int> mp;
  rep(i, 0, min(N, K - 1)) { mp[B[i]]++; }

  int x = 0;
  rep(i, 0, N) {
    dump(mp);
    dump(x, mp[x]);
    ans += mp[x];
    dump(ans);
    x++;
    x -= A[i];
    x = f(x);
    dump(x);
    mp[B[i]]--;
    if (i + K - 1 < N)
      mp[B[i + K - 1]]++;
  }
  cout << ans << endl;

  return 0;
}