Problem p02715 (C++)

Buggy submission (status: Runtime Error):
// https://atcoder.jp/contests/abc162/tasks/abc162_e
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i, n) FOR(i, 0, n)
#define REPR(i, n) for (int i = n - 1; i >= 0; i--)
#define FOR(i, s, n) for (int i = (s), i##_len = (n); i < i##_len; ++i)
#define ALL(obj) (obj).begin(), (obj).end()
#define ALLR(obj) (obj).rbegin(), (obj).rend()
#define DIV(a, b) ((a - 1) / b + 1)

template <int MOD> struct ModInt {
  static const int Mod = MOD;
  unsigned x;
  ModInt() : x(0) {}
  ModInt(signed sig) { x = sig < 0 ? sig % MOD + MOD : sig % MOD; }
  ModInt(signed long long sig) { x = sig < 0 ? sig % MOD + MOD : sig % MOD; }
  int get() const { return (int)x; }
  ModInt &operator+=(ModInt that) {
    if ((x += that.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator-=(ModInt that) {
    if ((x += MOD - that.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator*=(ModInt that) {
    x = (unsigned long long)x * that.x % MOD;
    return *this;
  }
  ModInt &operator/=(ModInt that) { return *this *= that.inverse(); }
  ModInt operator+(ModInt that) const { return ModInt(*this) += that; }
  ModInt operator-(ModInt that) const { return ModInt(*this) -= that; }
  ModInt operator*(ModInt that) const { return ModInt(*this) *= that; }
  ModInt operator/(ModInt that) const { return ModInt(*this) /= that; }
  ModInt inverse() const {
    long long a = x, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      std::swap(a, b);
      u -= t * v;
      std::swap(u, v);
    }
    return ModInt(u);
  }
  bool operator==(ModInt that) const { return x == that.x; }
  bool operator!=(ModInt that) const { return x != that.x; }
  ModInt operator-() const {
    ModInt t;
    t.x = x == 0 ? 0 : Mod - x;
    return t;
  }
};
template <int MOD> ostream &operator<<(ostream &st, const ModInt<MOD> a) {
  st << a.get();
  return st;
};
template <int MOD> ModInt<MOD> operator^(ModInt<MOD> a, unsigned long long k) {
  ModInt<MOD> r = 1;
  while (k) {
    if (k & 1)
      r *= a;
    a *= a;
    k >>= 1;
  }
  return r;
}
using mint = ModInt<(int)1e9 + 7>;

// https://www.hamayanhamayan.com/entry/2020/04/12/225411
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int N, K;
  cin >> N >> K;
  vector<mint> cnt(N + 1);
  for (int g = K; g >= 1; g--) {
    // ^ は XORではなくpower
    cnt[g] = mint(K / g) ^ N;
    int gg = g * 2;
    while (gg <= K) {
      cnt[g] -= cnt[gg];
      gg += g;
    }
  }
  mint ans = 0;
  FOR(i, 1, K + 1) { ans += cnt[i] * i; }
  cout << ans << endl;

  return 0;
}


Fixed submission:
// https://atcoder.jp/contests/abc162/tasks/abc162_e
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i, n) FOR(i, 0, n)
#define REPR(i, n) for (int i = n - 1; i >= 0; i--)
#define FOR(i, s, n) for (int i = (s), i##_len = (n); i < i##_len; ++i)
#define ALL(obj) (obj).begin(), (obj).end()
#define ALLR(obj) (obj).rbegin(), (obj).rend()
#define DIV(a, b) ((a - 1) / b + 1)

template <int MOD> struct ModInt {
  static const int Mod = MOD;
  unsigned x;
  ModInt() : x(0) {}
  ModInt(signed sig) { x = sig < 0 ? sig % MOD + MOD : sig % MOD; }
  ModInt(signed long long sig) { x = sig < 0 ? sig % MOD + MOD : sig % MOD; }
  int get() const { return (int)x; }
  ModInt &operator+=(ModInt that) {
    if ((x += that.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator-=(ModInt that) {
    if ((x += MOD - that.x) >= MOD)
      x -= MOD;
    return *this;
  }
  ModInt &operator*=(ModInt that) {
    x = (unsigned long long)x * that.x % MOD;
    return *this;
  }
  ModInt &operator/=(ModInt that) { return *this *= that.inverse(); }
  ModInt operator+(ModInt that) const { return ModInt(*this) += that; }
  ModInt operator-(ModInt that) const { return ModInt(*this) -= that; }
  ModInt operator*(ModInt that) const { return ModInt(*this) *= that; }
  ModInt operator/(ModInt that) const { return ModInt(*this) /= that; }
  ModInt inverse() const {
    long long a = x, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      std::swap(a, b);
      u -= t * v;
      std::swap(u, v);
    }
    return ModInt(u);
  }
  bool operator==(ModInt that) const { return x == that.x; }
  bool operator!=(ModInt that) const { return x != that.x; }
  ModInt operator-() const {
    ModInt t;
    t.x = x == 0 ? 0 : Mod - x;
    return t;
  }
};
template <int MOD> ostream &operator<<(ostream &st, const ModInt<MOD> a) {
  st << a.get();
  return st;
};
template <int MOD> ModInt<MOD> operator^(ModInt<MOD> a, unsigned long long k) {
  ModInt<MOD> r = 1;
  while (k) {
    if (k & 1)
      r *= a;
    a *= a;
    k >>= 1;
  }
  return r;
}
using mint = ModInt<(int)1e9 + 7>;

// https://www.hamayanhamayan.com/entry/2020/04/12/225411
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int N, K;
  cin >> N >> K;
  vector<mint> cnt(K + 1);
  for (int g = K; g >= 1; g--) {
    // ^ は XORではなくpower
    cnt[g] = mint(K / g) ^ N;
    int gg = g * 2;
    while (gg <= K) {
      cnt[g] -= cnt[gg];
      gg += g;
    }
  }
  mint ans = 0;
  FOR(i, 1, K + 1) { ans += cnt[i] * i; }
  cout << ans << endl;

  return 0;
}