Problem p02632 (C++)

Buggy submission (status: Runtime Error):
// 四則演算 #pragma GCC target("avx")
// 並列計算 #pragma GCC optimize("O3")
// 条件分岐を減らす #pragma GCC optimize("unroll-loops")
// 浮動小数点演算 #pragma GCC
// target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
template <class T> using PQ = priority_queue<T, vector<T>, greater<T>>;
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define eFOR(i, a, n) for (int i = (a); i <= (n); ++i)
#define rFOR(i, a, n) for (int i = (n)-1; i >= (a); --i)
#define erFOR(i, a, n) for (int i = (n); i >= (a); --i)
#define each(i, a) for (auto &i : a)
#define SORT(a) sort(a.begin(), a.end())
#define rSORT(a) sort(a.rbegin(), a.rend())
#define fSORT(a, f) sort(a.begin(), a.end(), f)
#define all(a) a.begin(), a.end()
#define out(y, x) ((y) < 0 || h <= (y) || (x) < 0 || w <= (x))
#define tp(a, i) get<i>(a)
#define line cout << "-----------------------------\n"
#define ENDL(i, n) ((i) == (n)-1 ? "\n" : " ")
#define stop system("pause")
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10;
constexpr ld pi = 3.1415926535897932;
template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
inline void init() {
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
template <class T> inline istream &operator>>(istream &is, vector<T> &v) {
  for (auto &a : v)
    is >> a;
  return is;
}
template <class T> inline istream &operator>>(istream &is, deque<T> &v) {
  for (auto &a : v)
    is >> a;
  return is;
}
template <class T, class U>
inline istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> inline vector<T> vec(size_t a) { return vector<T>(a); }
template <class T> inline vector<T> defvec(T def, size_t a) {
  return vector<T>(a, def);
}
template <class T, class... Ts> inline auto vec(size_t a, Ts... ts) {
  return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));
}
template <class T, class... Ts> inline auto defvec(T def, size_t a, Ts... ts) {
  return vector<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...));
}
template <class T> inline void print(const T &a) { cout << a << "\n"; }
template <class T, class... Ts> inline void print(const T &a, const Ts &...ts) {
  cout << a << " ";
  print(ts...);
}
template <class T> inline void print(const vector<T> &v) {
  for (int i = 0; i < v.size(); ++i)
    cout << v[i] << (i == v.size() - 1 ? "\n" : " ");
}
template <class T> inline void print(const vector<vector<T>> &v) {
  for (auto &a : v)
    print(a);
}
inline string reversed(const string &s) {
  string t = s;
  reverse(all(t));
  return t;
}

template <int modulo> struct ModInt {
  int x;

  ModInt() : x(0) {}
  ModInt(long long y)
      : x(y >= 0 ? y % modulo : (modulo - (-y) % modulo) % modulo) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= modulo)
      x -= modulo;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += modulo - p.x) >= modulo)
      x -= modulo;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % modulo);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = modulo, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    return ModInt(u);
  }

  ModInt pow(int e) {
    long long a = 1, p = x;
    while (e > 0) {
      if (e % 2 == 0) {
        p = (p * p) % modulo;
        e /= 2;
      } else {
        a = (a * p) % modulo;
        e--;
      }
    }
    return ModInt(a);
  }

  friend ostream &operator<<(ostream &os, const ModInt<modulo> &p) {
    return os << p.x;
  }
  friend istream &operator>>(istream &is, ModInt<modulo> &a) {
    long long x;
    is >> x;
    a = ModInt<modulo>(x);
    return (is);
  }
};
using modint = ModInt<mod>;

const int MAX = 510000;
ll fac[MAX], finv[MAX], inv[MAX];
bool Pre = true;
void cominit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, MAX) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
ll com(int n, int k) {
  if (Pre) {
    cominit();
    Pre = false;
  }
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}

int main() {
  init();

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

  modint ans = 0;
  FOR(i, n - 1, n + k) {
    ans += modint(com(i, n - 1)) * modint(25).pow(i - n + 1) *
           modint(26).pow(n + k - i - 1);
  }
  print(ans);

  return 0;
}

Fixed submission:
// 四則演算 #pragma GCC target("avx")
// 並列計算 #pragma GCC optimize("O3")
// 条件分岐を減らす #pragma GCC optimize("unroll-loops")
// 浮動小数点演算 #pragma GCC
// target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
template <class T> using PQ = priority_queue<T, vector<T>, greater<T>>;
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define eFOR(i, a, n) for (int i = (a); i <= (n); ++i)
#define rFOR(i, a, n) for (int i = (n)-1; i >= (a); --i)
#define erFOR(i, a, n) for (int i = (n); i >= (a); --i)
#define each(i, a) for (auto &i : a)
#define SORT(a) sort(a.begin(), a.end())
#define rSORT(a) sort(a.rbegin(), a.rend())
#define fSORT(a, f) sort(a.begin(), a.end(), f)
#define all(a) a.begin(), a.end()
#define out(y, x) ((y) < 0 || h <= (y) || (x) < 0 || w <= (x))
#define tp(a, i) get<i>(a)
#define line cout << "-----------------------------\n"
#define ENDL(i, n) ((i) == (n)-1 ? "\n" : " ")
#define stop system("pause")
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10;
constexpr ld pi = 3.1415926535897932;
template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
inline void init() {
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
template <class T> inline istream &operator>>(istream &is, vector<T> &v) {
  for (auto &a : v)
    is >> a;
  return is;
}
template <class T> inline istream &operator>>(istream &is, deque<T> &v) {
  for (auto &a : v)
    is >> a;
  return is;
}
template <class T, class U>
inline istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <class T> inline vector<T> vec(size_t a) { return vector<T>(a); }
template <class T> inline vector<T> defvec(T def, size_t a) {
  return vector<T>(a, def);
}
template <class T, class... Ts> inline auto vec(size_t a, Ts... ts) {
  return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));
}
template <class T, class... Ts> inline auto defvec(T def, size_t a, Ts... ts) {
  return vector<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...));
}
template <class T> inline void print(const T &a) { cout << a << "\n"; }
template <class T, class... Ts> inline void print(const T &a, const Ts &...ts) {
  cout << a << " ";
  print(ts...);
}
template <class T> inline void print(const vector<T> &v) {
  for (int i = 0; i < v.size(); ++i)
    cout << v[i] << (i == v.size() - 1 ? "\n" : " ");
}
template <class T> inline void print(const vector<vector<T>> &v) {
  for (auto &a : v)
    print(a);
}
inline string reversed(const string &s) {
  string t = s;
  reverse(all(t));
  return t;
}

template <int modulo> struct ModInt {
  int x;

  ModInt() : x(0) {}
  ModInt(long long y)
      : x(y >= 0 ? y % modulo : (modulo - (-y) % modulo) % modulo) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= modulo)
      x -= modulo;
    return *this;
  }
  ModInt &operator-=(const ModInt &p) {
    if ((x += modulo - p.x) >= modulo)
      x -= modulo;
    return *this;
  }
  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % modulo);
    return *this;
  }
  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }
  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }
  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = modulo, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    return ModInt(u);
  }

  ModInt pow(int e) {
    long long a = 1, p = x;
    while (e > 0) {
      if (e % 2 == 0) {
        p = (p * p) % modulo;
        e /= 2;
      } else {
        a = (a * p) % modulo;
        e--;
      }
    }
    return ModInt(a);
  }

  friend ostream &operator<<(ostream &os, const ModInt<modulo> &p) {
    return os << p.x;
  }
  friend istream &operator>>(istream &is, ModInt<modulo> &a) {
    long long x;
    is >> x;
    a = ModInt<modulo>(x);
    return (is);
  }
};
using modint = ModInt<mod>;

const int MAX = 2000001;
ll fac[MAX], finv[MAX], inv[MAX];
bool Pre = true;
void cominit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  FOR(i, 2, MAX) {
    fac[i] = fac[i - 1] * i % mod;
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
ll com(int n, int k) {
  if (Pre) {
    cominit();
    Pre = false;
  }
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}

int main() {
  init();

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

  modint ans = 0;
  FOR(i, n - 1, n + k) {
    ans += modint(com(i, n - 1)) * modint(25).pow(i - n + 1) *
           modint(26).pow(n + k - i - 1);
  }
  print(ans);

  return 0;
}