Problem p02632 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdint>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

#ifdef _DEBUG
#define DUMP(x) std::cerr << (#x) << " = " << (x) << "\n"
#else
#define DUMP(x)
#endif
#define REP(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define EREP(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define RREP(i, a, b) for (int i = (int)(a)-1; i >= (int)(b); --i)
#define rep(i, n) REP(i, 0, n)
#define erep(i, n) EREP(i, 0, n)
#define rrep(i, n) RREP(i, n, 0)
#define ALL(r) (r).begin(), (r).end()
#define pb push_back
#define eb emplace_back
#define fi first
#define se second

template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  os << "{";
  rep(i, v.size()) os << v[i] << (i == (int)v.size() - 1 ? "" : ", ");
  os << "}";
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &p) {
  return (os << "(" << p.first << ", " << p.second << ")");
}
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &m) {
  bool first = true;
  os << "{";
  for (const auto &e : m) {
    if (!first)
      os << ", ";
    os << "{" << e.first << ": " << e.second << "}";
    first = false;
  }
  os << "}";
  return os;
}
template <typename T> ostream &operator<<(ostream &os, set<T> &s) {
  os << "{";
  bool first = true;
  for (const auto &e : s) {
    if (!first)
      os << ", ";
    os << e;
    first = false;
  }
  os << "}";
  return os;
}
template <typename T> T dup(T x, T y) { return (x + y - 1) / y; };
template <typename A, size_t N, typename T>
inline void arrayFill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
struct in {
  const size_t n = 0;
  in() = default;
  in(size_t n) : n(n){};
  template <typename T> operator T() {
    T ret;
    cin >> ret;
    return ret;
  }
  template <typename T> operator vector<T>() {
    assert(n != 0);
    vector<T> ret(n);
    for (T &x : ret) {
      T tmp = in();
      x = tmp;
    }
    return ret;
  }
  template <typename T, typename U> operator pair<T, U>() {
    pair<T, U> ret;
    ret.first = in();
    ret.second = in();
    return ret;
  }
};
template <typename T> inline void out(const T x) { std::cout << x << '\n'; };

using ll = int64_t;
using vint = vector<int32_t>;
using vvint = vector<vint>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vstr = vector<string>;
using pint = pair<int32_t, int32_t>;
using vpint = vector<pint>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
using setint = set<int32_t>;
using qint = queue<int32_t>;
using qpint = queue<pint>;

constexpr std::int32_t INF = 1001001001;
constexpr std::int64_t LINF = 1001001001001001001;

constexpr int mod = 1'000'000'007;
// constexpr int mod = 998'244'353;
class mint {
private:
  std::int64_t m_x;

public:
  mint(const std::int64_t m_x = 0) : m_x((m_x % mod + mod) % mod) {}
  std::int64_t get() const { return m_x; }
  mint operator-() const { return mint(-m_x); }
  bool operator==(const mint a) const { return m_x == a.get(); }
  bool operator!=(const mint a) const { return !(*this == a); }
  mint &operator+=(const mint a) {
    if ((m_x += a.m_x) >= mod)
      m_x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((m_x += mod - a.m_x) >= mod)
      m_x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (m_x *= a.m_x) %= mod;
    return *this;
  }
  mint &operator++() { return *this += 1; }
  mint operator++(int) {
    mint tmp = *this;
    ++*this;
    return tmp;
  }
  mint operator--() { return *this -= 1; }
  mint operator--(int) {
    mint tmp = *this;
    --*this;
    return tmp;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(const std::int64_t t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};
istream &operator>>(istream &is, mint &a) {
  ll x;
  is >> x;
  a = x;
  return is;
}
ostream &operator<<(ostream &os, const mint &a) {
  os << a.get();
  return os;
}

// require modint
struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
} comb(200005);

void Main() {
  int k = in();
  string s = in();
  int n = s.size();

  mint ans = 0;
  erep(i, k) {
    ans += comb(n - 1 + k - i, n - 1) * mint(25).pow(k - i) * mint(26).pow(i);
  }
  out(ans);
}

signed main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(15);
  Main();
  return 0;
}

Fixed submission:
#include <algorithm>
#include <array>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdint>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

#ifdef _DEBUG
#define DUMP(x) std::cerr << (#x) << " = " << (x) << "\n"
#else
#define DUMP(x)
#endif
#define REP(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define EREP(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define RREP(i, a, b) for (int i = (int)(a)-1; i >= (int)(b); --i)
#define rep(i, n) REP(i, 0, n)
#define erep(i, n) EREP(i, 0, n)
#define rrep(i, n) RREP(i, n, 0)
#define ALL(r) (r).begin(), (r).end()
#define pb push_back
#define eb emplace_back
#define fi first
#define se second

template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  os << "{";
  rep(i, v.size()) os << v[i] << (i == (int)v.size() - 1 ? "" : ", ");
  os << "}";
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &p) {
  return (os << "(" << p.first << ", " << p.second << ")");
}
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &m) {
  bool first = true;
  os << "{";
  for (const auto &e : m) {
    if (!first)
      os << ", ";
    os << "{" << e.first << ": " << e.second << "}";
    first = false;
  }
  os << "}";
  return os;
}
template <typename T> ostream &operator<<(ostream &os, set<T> &s) {
  os << "{";
  bool first = true;
  for (const auto &e : s) {
    if (!first)
      os << ", ";
    os << e;
    first = false;
  }
  os << "}";
  return os;
}
template <typename T> T dup(T x, T y) { return (x + y - 1) / y; };
template <typename A, size_t N, typename T>
inline void arrayFill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
struct in {
  const size_t n = 0;
  in() = default;
  in(size_t n) : n(n){};
  template <typename T> operator T() {
    T ret;
    cin >> ret;
    return ret;
  }
  template <typename T> operator vector<T>() {
    assert(n != 0);
    vector<T> ret(n);
    for (T &x : ret) {
      T tmp = in();
      x = tmp;
    }
    return ret;
  }
  template <typename T, typename U> operator pair<T, U>() {
    pair<T, U> ret;
    ret.first = in();
    ret.second = in();
    return ret;
  }
};
template <typename T> inline void out(const T x) { std::cout << x << '\n'; };

using ll = int64_t;
using vint = vector<int32_t>;
using vvint = vector<vint>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vstr = vector<string>;
using pint = pair<int32_t, int32_t>;
using vpint = vector<pint>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
using setint = set<int32_t>;
using qint = queue<int32_t>;
using qpint = queue<pint>;

constexpr std::int32_t INF = 1001001001;
constexpr std::int64_t LINF = 1001001001001001001;

constexpr int mod = 1'000'000'007;
// constexpr int mod = 998'244'353;
class mint {
private:
  std::int64_t m_x;

public:
  mint(const std::int64_t m_x = 0) : m_x((m_x % mod + mod) % mod) {}
  std::int64_t get() const { return m_x; }
  mint operator-() const { return mint(-m_x); }
  bool operator==(const mint a) const { return m_x == a.get(); }
  bool operator!=(const mint a) const { return !(*this == a); }
  mint &operator+=(const mint a) {
    if ((m_x += a.m_x) >= mod)
      m_x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((m_x += mod - a.m_x) >= mod)
      m_x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (m_x *= a.m_x) %= mod;
    return *this;
  }
  mint &operator++() { return *this += 1; }
  mint operator++(int) {
    mint tmp = *this;
    ++*this;
    return tmp;
  }
  mint operator--() { return *this -= 1; }
  mint operator--(int) {
    mint tmp = *this;
    --*this;
    return tmp;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }

  mint pow(const std::int64_t t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return (*this) *= a.inv(); }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};
istream &operator>>(istream &is, mint &a) {
  ll x;
  is >> x;
  a = x;
  return is;
}
ostream &operator<<(ostream &os, const mint &a) {
  os << a.get();
  return os;
}

// require modint
struct combination {
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1) {
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
} comb(2000005);

void Main() {
  int k = in();
  string s = in();
  int n = s.size();

  mint ans = 0;
  erep(i, k) {
    ans += comb(n - 1 + k - i, n - 1) * mint(25).pow(k - i) * mint(26).pow(i);
  }
  out(ans);
}

signed main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(15);
  Main();
  return 0;
}