Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using P = pair<lint, lint>;
#define FOR(i, begin, end)                                                     \
  for (int i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \
  for (int i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define ALL(a) (a).begin(), (a).end()
constexpr int MOD = 1000000007;
constexpr int INF = 2147483647;
void yes(bool expr) { cout << (expr ? "Yes" : "No") << "\n"; }
constexpr int MAX = 200001;
lint fact[MAX], finv[MAX], inv[MAX];
void cominit() {
  fact[0] = fact[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, MAX) {
    fact[i] = fact[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
lint power(lint x, lint n, lint mod) {
  lint ret = 1;
  while (n > 0) {
    if (n & 1)
      (ret *= x) %= mod;
    (x *= x) %= mod;
    n >>= 1;
  }
  return ret;
}
lint ncr(lint n, lint r, lint mod) {
  if (n < 0 || r < 0 || n - r < 0)
    return 0;
  return fact[n] * finv[r] % mod * finv[n - r] % mod;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  lint K;
  cin >> K;
  string S;
  cin >> S;
  cominit();
  lint ans = 0;
  REP(i, K + 1) {
    ans = (ans + ncr(S.size() + i - 1, S.size() - 1, MOD) *
                     power(26, K - i, MOD) % MOD * power(25, i, MOD) % MOD) %
          MOD;
  }
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using P = pair<lint, lint>;
#define FOR(i, begin, end)                                                     \
  for (int i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \
  for (int i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define ALL(a) (a).begin(), (a).end()
constexpr int MOD = 1000000007;
constexpr int INF = 2147483647;
void yes(bool expr) { cout << (expr ? "Yes" : "No") << "\n"; }
constexpr int MAX = 2000001;
lint fact[MAX], finv[MAX], inv[MAX];
void cominit() {
  fact[0] = fact[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, MAX) {
    fact[i] = fact[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
lint power(lint x, lint n, lint mod) {
  lint ret = 1;
  while (n > 0) {
    if (n & 1)
      (ret *= x) %= mod;
    (x *= x) %= mod;
    n >>= 1;
  }
  return ret;
}
lint ncr(lint n, lint r, lint mod) {
  if (n < 0 || r < 0 || n - r < 0)
    return 0;
  return fact[n] * finv[r] % mod * finv[n - r] % mod;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  lint K;
  cin >> K;
  string S;
  cin >> S;
  cominit();
  lint ans = 0;
  REP(i, K + 1) {
    ans = (ans + ncr(S.size() + i - 1, S.size() - 1, MOD) *
                     power(26, K - i, MOD) % MOD * power(25, i, MOD) % MOD) %
          MOD;
  }
  cout << ans << endl;
}