Problem p02851 (C++)

Buggy submission (status: Runtime Error):
/*input
5 4
1 4 2 3 5
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
#pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
typedef long long ll;
#define ld double
#define pii pair<ll, ll>
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
const ll maxn = 1e5 + 5;
const ll maxlg = __lg(maxn) + 2;
const ll INF64 = 4e18;
const int INF = 0x3f3f3f3f;
const ll MOD = ll(1e9 + 7);
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
ll mult(ll a, ll b) { return ((a % MOD) * (b % MOD)) % MOD; }
ll mypow(ll a, ll b, ll MOD) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
    b >>= 1;
  }
  return res;
}
int pre[maxn];
map<int, int> m;
int32_t main() {
  int n, k;
  cin >> n >> k;
  ll ans = 0;
  m[0]++;
  for (int i = 1, x; i <= n; i++) {
    cin >> x;
    pre[i] = ((pre[i - 1] + x - 1) % k + k) % k;
    if (i >= k)
      m[pre[i - k]]--;
    ans += m[pre[i]];
    m[pre[i]]++;
  }
  cout << ans << endl;
}

Fixed submission:
/*input
5 4
1 4 2 3 5
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
#pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
typedef long long ll;
#define ld double
#define pii pair<ll, ll>
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
const ll maxn = 2e5 + 5;
const ll maxlg = __lg(maxn) + 2;
const ll INF64 = 4e18;
const int INF = 0x3f3f3f3f;
const ll MOD = ll(1e9 + 7);
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
ll mult(ll a, ll b) { return ((a % MOD) * (b % MOD)) % MOD; }
ll mypow(ll a, ll b, ll MOD) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
    b >>= 1;
  }
  return res;
}
int pre[maxn];
map<int, int> m;
int32_t main() {
  int n, k;
  cin >> n >> k;
  ll ans = 0;
  m[0]++;
  for (int i = 1, x; i <= n; i++) {
    cin >> x;
    pre[i] = ((pre[i - 1] + x - 1) % k + k) % k;
    if (i >= k)
      m[pre[i - k]]--;
    ans += m[pre[i]];
    m[pre[i]]++;
  }
  cout << ans << endl;
}