Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(), x.end()
#define rep(i, n) for (int i = 0; i < (n); i++)
#define debug(v)                                                               \
  cout << #v << ":";                                                           \
  for (auto x : v) {                                                           \
    cout << x << ' ';                                                          \
  }                                                                            \
  cout << endl;
#define INF 1000000000
#define mod 1000000007
using ll = long long;
const ll LINF = 1001002003004005006ll;
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
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;
}

const int MAX = 1010000;
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 or k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}
ll fact_mod(ll n) {
  ll ret = 1;
  for (ll i = 2; i <= n; i++)
    ret = ret * (i % mod) % mod;
  return ret;
}
ll pow_mod(ll x, ll n) {
  if (n == 0)
    return 1;
  ll ret = pow_mod((x * x) % mod, n / 2);
  if (n & 1)
    ret = (ret * x) % mod;
  return ret;
}
// O(r)
ll combination_mod(ll n, ll r) {
  if (r > n - r)
    r = n - r;
  if (r == 0)
    return 1;
  ll a = 1;
  for (ll i = 0; i < r; i++)
    a = a * ((n - i) % mod) % mod;
  ll b = pow_mod(fact_mod(r), mod - 2);
  return (a % mod) * (b % mod) % mod;
}
ll inv_mod(ll n) { return pow_mod(n, mod - 2); }

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(0);

  COMinit();

  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = (int)s.size();

  ll ans = 0;

  rep(i, k + 1) {
    ll rem = k - i;
    ll res = pow_mod(26, i) * COM(n - 2 + rem + 1, rem) % mod *
             pow_mod(25, rem) % mod;
    ans += res;
    ans %= mod;
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(), x.end()
#define rep(i, n) for (int i = 0; i < (n); i++)
#define debug(v)                                                               \
  cout << #v << ":";                                                           \
  for (auto x : v) {                                                           \
    cout << x << ' ';                                                          \
  }                                                                            \
  cout << endl;
#define INF 1000000000
#define mod 1000000007
using ll = long long;
const ll LINF = 1001002003004005006ll;
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
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;
}

const int MAX = 4010000;
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 or k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}
ll fact_mod(ll n) {
  ll ret = 1;
  for (ll i = 2; i <= n; i++)
    ret = ret * (i % mod) % mod;
  return ret;
}
ll pow_mod(ll x, ll n) {
  if (n == 0)
    return 1;
  ll ret = pow_mod((x * x) % mod, n / 2);
  if (n & 1)
    ret = (ret * x) % mod;
  return ret;
}
// O(r)
ll combination_mod(ll n, ll r) {
  if (r > n - r)
    r = n - r;
  if (r == 0)
    return 1;
  ll a = 1;
  for (ll i = 0; i < r; i++)
    a = a * ((n - i) % mod) % mod;
  ll b = pow_mod(fact_mod(r), mod - 2);
  return (a % mod) * (b % mod) % mod;
}
ll inv_mod(ll n) { return pow_mod(n, mod - 2); }

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(0);

  COMinit();

  int k;
  cin >> k;
  string s;
  cin >> s;
  int n = (int)s.size();

  ll ans = 0;

  rep(i, k + 1) {
    ll rem = k - i;
    ll res = pow_mod(26, i) * COM(n - 2 + rem + 1, rem) % mod *
             pow_mod(25, rem) % mod;
    ans += res;
    ans %= mod;
  }
  cout << ans << endl;
  return 0;
}