Problem p03963 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;

#define int long long // 禁忌
using ll = long long;
using lld = long double;
#define P pair<ll, ll>
#define Vi vector<ll>
#define VVi vector<vector<ll>>
#define Vd vector<double>
#define Vb vector<bool>
#define Vs vector<string>
#define Vc vector<char>
#define M map<ll, ll>
#define S set<ll>
#define PQ priority_queue<ll>
#define PQG priority_queue < ll, V, greater<ll>

// 重み付き辺
template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

// 重み付き辺集合
template <typename T> using Edges = vector<edge<T>>;

// 重み付きグラフ
template <typename T> using WeightedGraph = vector<Edges<T>>;

using UnWeightedGraph = VVi; // 重みなしグラフ

// 距離行列
template <typename T> using Matrix = vector<vector<T>>;

const int MOD = 1000000007;
const int INF = 1061109567;
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define PER(i, n) for (int i = (n - 1); i >= 0; --i)
#define ALL(V) (V).begin(), (V).end()
#define SORT(V) sort(ALL(V))   // 小さい方からソート
#define REV(V) reverse(ALL(V)) // リバース
#define RSORT(V)                                                               \
  SORT(V);                                                                     \
  REV(V)                                 // 大きい方からソート
#define NEXP(V) next_permutation(ALL(V)) // 順列
#define pb(n) emplace_back(n)
#define popb pop_back()
#define endl '\n'
#define Endl '\n'
#define DUMP(x) cout << #x << " = " << (x) << endl

// UnionFind
class UnionFind {
public:
  vector<int> par;
  vector<int> siz;

  UnionFind(int sz_) : par(sz_), siz(sz_, 1LL) {
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }

  int root(int x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  int size(int x) { return siz[root(x)]; }
};

// modint
template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    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 = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};
using modint = ModInt<MOD>;

// dijkstra O(E logV)
template <typename T> vector<T> dijkstra(WeightedGraph<T> &g, int s) {
  const auto INF2 = numeric_limits<T>::max();
  vector<T> dist(g.size(), INF2);
  using Pi = pair<T, int>;
  priority_queue<Pi, vector<Pi>, greater<Pi>> que;
  dist[s] = 0;
  que.emplace(dist[s], s);
  while (!que.empty()) {
    T cost;
    int idx;
    tie(cost, idx) = que.top();
    que.pop();
    if (dist[idx] < cost)
      continue;
    for (auto &e : g[idx]) {
      auto next_cost = cost + e.cost;
      if (dist[e.to] <= next_cost)
        continue;
      dist[e.to] = next_cost;
      que.emplace(dist[e.to], e.to);
    }
  }
  return dist;
}

// 負の辺を持つ単一始点最短路 O(VE)
template <typename T> vector<T> bellman_ford(Edges<T> &edges, int V, int s) {
  const auto INF2 = numeric_limits<T>::max();
  vector<T> dist(V, INF2);
  dist[s] = 0;
  REP(i, (V - 1)) {
    for (auto &e : edges) {
      if (dist[e.src] == INF2)
        continue;
      dist[e.to] = min(dist[e.to], dist[e.src] + e.cost);
    }
  }
  for (auto &e : edges) {
    if (dist[e.src] == INF2)
      continue;
    if (dist[e.src] + e.cost < dist[e.to])
      return vector<T>();
  }
  return dist;
}

// 全点対間最短路 O(V^3)
template <typename T> void warshall_floyd(Matrix<T> &g, T lINF) {
  ll n = g.size();
  REP(k, n) {
    REP(i, n) {
      REP(j, n) {
        if (g[i][k] == lINF || g[k][j] == lINF)
          continue;
        g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
      }
    }
  }
  return;
}

// 入出力高速化
void fast(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  return;
}

// vectorの合計値を返します
ll vsum(Vi V) {
  ll res = 0LL;
  ll size = V.size();
  REP(i, size) { res += V.at(i); }
  return res;
}

// vectorの平均値を返します
lld vave(Vi V) {
  lld size = V.size();
  return (lld)vsum(V) / size;
}

// vectorの最大値を返します
ll vmax(Vi V) {
  RSORT(V);
  return V.at(0);
}

// vectorの最小値を返します
ll vmin(Vi V) {
  SORT(V);
  return V.at(0);
}

// 数値 b が a より大きく、 c より小さいことを判定します
template <typename T> bool mid1(T a, T b, T c) { return (a < b) && (b < c); }

// 数値 b が a以上 で、c以下 であることを判定します
template <typename T> bool mid2(T a, T b, T c) { return (a <= b) && (b <= c); }

// YES or NO
template <typename T> void YES(T n) { cout << ((n) ? "YES" : "NO") << endl; }

// Yes or No
template <typename T> void Yes(T n) { cout << ((n) ? "Yes" : "No") << endl; }

// yes or no
template <typename T> void yes(T n) { cout << ((n) ? "yes" : "no") << endl; }

// Yay! or :(
template <typename T> void Yay(T n) { cout << ((n) ? "Yay!" : ":(") << endl; }

// 標準入力します
inline void in(void) { return; }
template <typename First, typename... Rest>
void in(First &first, Rest &...rest) {
  cin >> first;
  in(rest...);
  return;
}

// vectorを標準入力
template <typename T> void vin(T &V) {
  ll size = V.size();
  REP(i, size) { cin >> V[i]; }
  return;
}

// 標準出力します
inline void out(void) {
  cout << endl;
  return;
}
template <typename T> void out(T only) {
  cout << only << endl;
  return;
}
template <typename First, typename... Rest>
void out(First first, Rest... rest) {
  cout << first << " ";
  out(rest...);
  return;
}

// vectorを標準出力
template <typename T> void vout(T &V) {
  ll size = V.size();
  REP(i, size) { cout << V.at(i) << endl; }
}

// vectorを１行に標準出力
template <typename T> void vout2(T &V) {
  ll size = V.size();
  REP(i, size) { cout << V[i] << " \n"[i == (size - 1)]; }
}

// 最大公約数を求めます
int gcd(int a, int b) { return b != 0 ? gcd(b, a % b) : a; }

// 最小公倍数を求めます
int lcm(int a, int b) { return a * b / gcd(a, b); }

// 文字を全て大文字にします
string toStrUp(string str) {
  char diff = 'A' - 'a';
  REP(i, str.size()) str[i] += diff;
  return str;
}

// 素因数分解します (O(sqrt(n)))
map<int, int> prime_factor(int n) {
  map<int, int> ret;
  for (int i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if (n != 1)
    ret[n] = 1;
  return ret;
}

// 素数判定 (O(sqrt(n)))
bool is_prime(int x) {
  for (int i = 2; i * i <= x; i++) {
    if (x % i == 0)
      return false;
  }
  return true;
}

// 進数変換 (O(log n))
template <typename T> vector<T> convert_base(T x, T b) {
  vector<T> ret;
  T t = 1, k = abs(b);
  while (x) {
    ret.emplace_back((x * t) % k);
    if (ret.back() < 0)
      ret.back() += k;
    x -= ret.back() * t;
    x /= k;
    t *= b / k;
  }
  if (ret.empty())
    ret.emplace_back(0);
  reverse(begin(ret), end(ret));
  return ret;
}

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

signed main() {
  fast();
  // 使えない変数名
  // P, M, S, PQ, PQG
  // ここから
  int n, m, k;
  string s;
  in(n, k);
  int ans = k;
  REP(i, n - 1) { ans *= (k - 1); }
  out(ans);
}

Fixed submission:
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;

#define int long long // 禁忌
using ll = long long;
using lld = long double;
#define P pair<ll, ll>
#define Vi vector<ll>
#define VVi vector<vector<ll>>
#define Vd vector<double>
#define Vb vector<bool>
#define Vs vector<string>
#define Vc vector<char>
#define M map<ll, ll>
#define S set<ll>
#define PQ priority_queue<ll>
#define PQG priority_queue < ll, V, greater<ll>

// 重み付き辺
template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

// 重み付き辺集合
template <typename T> using Edges = vector<edge<T>>;

// 重み付きグラフ
template <typename T> using WeightedGraph = vector<Edges<T>>;

using UnWeightedGraph = VVi; // 重みなしグラフ

// 距離行列
template <typename T> using Matrix = vector<vector<T>>;

const int MOD = 1000000007;
const int INF = 1061109567;
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define PER(i, n) for (int i = (n - 1); i >= 0; --i)
#define ALL(V) (V).begin(), (V).end()
#define SORT(V) sort(ALL(V))   // 小さい方からソート
#define REV(V) reverse(ALL(V)) // リバース
#define RSORT(V)                                                               \
  SORT(V);                                                                     \
  REV(V)                                 // 大きい方からソート
#define NEXP(V) next_permutation(ALL(V)) // 順列
#define pb(n) emplace_back(n)
#define popb pop_back()
#define endl '\n'
#define Endl '\n'
#define DUMP(x) cout << #x << " = " << (x) << endl

// UnionFind
class UnionFind {
public:
  vector<int> par;
  vector<int> siz;

  UnionFind(int sz_) : par(sz_), siz(sz_, 1LL) {
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }

  int root(int x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  int size(int x) { return siz[root(x)]; }
};

// modint
template <int mod> struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if ((x += mod - p.x) >= mod)
      x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % mod);
    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 = mod, u = 1, v = 0, t;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while (n > 0) {
      if (n & 1)
        ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt<mod>(t);
    return (is);
  }

  static int get_mod() { return mod; }
};
using modint = ModInt<MOD>;

// dijkstra O(E logV)
template <typename T> vector<T> dijkstra(WeightedGraph<T> &g, int s) {
  const auto INF2 = numeric_limits<T>::max();
  vector<T> dist(g.size(), INF2);
  using Pi = pair<T, int>;
  priority_queue<Pi, vector<Pi>, greater<Pi>> que;
  dist[s] = 0;
  que.emplace(dist[s], s);
  while (!que.empty()) {
    T cost;
    int idx;
    tie(cost, idx) = que.top();
    que.pop();
    if (dist[idx] < cost)
      continue;
    for (auto &e : g[idx]) {
      auto next_cost = cost + e.cost;
      if (dist[e.to] <= next_cost)
        continue;
      dist[e.to] = next_cost;
      que.emplace(dist[e.to], e.to);
    }
  }
  return dist;
}

// 負の辺を持つ単一始点最短路 O(VE)
template <typename T> vector<T> bellman_ford(Edges<T> &edges, int V, int s) {
  const auto INF2 = numeric_limits<T>::max();
  vector<T> dist(V, INF2);
  dist[s] = 0;
  REP(i, (V - 1)) {
    for (auto &e : edges) {
      if (dist[e.src] == INF2)
        continue;
      dist[e.to] = min(dist[e.to], dist[e.src] + e.cost);
    }
  }
  for (auto &e : edges) {
    if (dist[e.src] == INF2)
      continue;
    if (dist[e.src] + e.cost < dist[e.to])
      return vector<T>();
  }
  return dist;
}

// 全点対間最短路 O(V^3)
template <typename T> void warshall_floyd(Matrix<T> &g, T lINF) {
  ll n = g.size();
  REP(k, n) {
    REP(i, n) {
      REP(j, n) {
        if (g[i][k] == lINF || g[k][j] == lINF)
          continue;
        g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
      }
    }
  }
  return;
}

// 入出力高速化
void fast(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  return;
}

// vectorの合計値を返します
ll vsum(Vi V) {
  ll res = 0LL;
  ll size = V.size();
  REP(i, size) { res += V.at(i); }
  return res;
}

// vectorの平均値を返します
lld vave(Vi V) {
  lld size = V.size();
  return (lld)vsum(V) / size;
}

// vectorの最大値を返します
ll vmax(Vi V) {
  RSORT(V);
  return V.at(0);
}

// vectorの最小値を返します
ll vmin(Vi V) {
  SORT(V);
  return V.at(0);
}

// 数値 b が a より大きく、 c より小さいことを判定します
template <typename T> bool mid1(T a, T b, T c) { return (a < b) && (b < c); }

// 数値 b が a以上 で、c以下 であることを判定します
template <typename T> bool mid2(T a, T b, T c) { return (a <= b) && (b <= c); }

// YES or NO
template <typename T> void YES(T n) { cout << ((n) ? "YES" : "NO") << endl; }

// Yes or No
template <typename T> void Yes(T n) { cout << ((n) ? "Yes" : "No") << endl; }

// yes or no
template <typename T> void yes(T n) { cout << ((n) ? "yes" : "no") << endl; }

// Yay! or :(
template <typename T> void Yay(T n) { cout << ((n) ? "Yay!" : ":(") << endl; }

// 標準入力します
inline void in(void) { return; }
template <typename First, typename... Rest>
void in(First &first, Rest &...rest) {
  cin >> first;
  in(rest...);
  return;
}

// vectorを標準入力
template <typename T> void vin(T &V) {
  ll size = V.size();
  REP(i, size) { cin >> V[i]; }
  return;
}

// 標準出力します
inline void out(void) {
  cout << endl;
  return;
}
template <typename T> void out(T only) {
  cout << only << endl;
  return;
}
template <typename First, typename... Rest>
void out(First first, Rest... rest) {
  cout << first << " ";
  out(rest...);
  return;
}

// vectorを標準出力
template <typename T> void vout(T &V) {
  ll size = V.size();
  REP(i, size) { cout << V.at(i) << endl; }
}

// vectorを１行に標準出力
template <typename T> void vout2(T &V) {
  ll size = V.size();
  REP(i, size) { cout << V[i] << " \n"[i == (size - 1)]; }
}

// 最大公約数を求めます
int gcd(int a, int b) { return b != 0 ? gcd(b, a % b) : a; }

// 最小公倍数を求めます
int lcm(int a, int b) { return a * b / gcd(a, b); }

// 文字を全て大文字にします
string toStrUp(string str) {
  char diff = 'A' - 'a';
  REP(i, str.size()) str[i] += diff;
  return str;
}

// 素因数分解します (O(sqrt(n)))
map<int, int> prime_factor(int n) {
  map<int, int> ret;
  for (int i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if (n != 1)
    ret[n] = 1;
  return ret;
}

// 素数判定 (O(sqrt(n)))
bool is_prime(int x) {
  for (int i = 2; i * i <= x; i++) {
    if (x % i == 0)
      return false;
  }
  return true;
}

// 進数変換 (O(log n))
template <typename T> vector<T> convert_base(T x, T b) {
  vector<T> ret;
  T t = 1, k = abs(b);
  while (x) {
    ret.emplace_back((x * t) % k);
    if (ret.back() < 0)
      ret.back() += k;
    x -= ret.back() * t;
    x /= k;
    t *= b / k;
  }
  if (ret.empty())
    ret.emplace_back(0);
  reverse(begin(ret), end(ret));
  return ret;
}

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

signed main() {
  fast();
  // 使えない変数名
  // P, M, S, PQ, PQG
  // ここから
  int n, m, k;
  string s;
  in(n, k);
  out(k * (int)(pow(k - 1, n - 1)));
}