Problem p02596 (C++)

Buggy submission (status: Time Limit Exceeded):
#define LOCAL
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;
using ll = long long;
using P = pair<int, int>;

#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define ALL(x) ((x).begin(), (x).end())
#define SIZE(x) ((int)(x).size())
#define BITS(x) (1LL << (x))

constexpr int INF = 1001001001;
constexpr ll LINF = 1001001001001001001LL;
constexpr int MOD = 1000000007;
constexpr double EPS = 1e-10;
constexpr double PI = 3.141592653589793;
constexpr int dx[4] = {1, 0, -1, 0};
constexpr int dy[4] = {0, 1, 0, -1};
constexpr int dx8[8] = {1, 1, 1, 0, 0, -1, -1, -1};
constexpr int dy8[8] = {1, 0, -1, 1, -1, 1, 0, -1};

random_device seed_gen;
mt19937 mrand(seed_gen());
mt19937_64 mrand64(seed_gen());
int rand32(int x) { return mrand() % x; }
ll rand64(long x) { return mrand64() % x; }

template <typename A, typename B>
ostream &operator<<(ostream &out, const pair<A, B> &a) {
  out << "(" << a.first << "," << a.second << ")";
  return out;
}
template <typename T, size_t N>
ostream &operator<<(ostream &out, const array<T, N> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ",");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ",");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T, class Cmp>
ostream &operator<<(ostream &out, const set<T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ",");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream &operator<<(ostream &out, const map<U, T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &p : a) {
    out << (first ? "" : ",");
    out << p.first << ": " << p.second;
    first = 0;
  }
  out << "}";
  return out;
}
#ifdef LOCAL
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...) 42
#endif
template <typename Arg1> void __f(const char *name, Arg1 &&arg1) {
  cerr << name << ": " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char *names, Arg1 &&arg1, Args &&...args) {
  const char *comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << ": " << arg1 << " |";
  __f(comma + 1, args...);
}

struct fast_ios {
  static constexpr int IOS_PREC = 15;
  static constexpr bool AUTOFLUSH = false;

  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(IOS_PREC);
    if (AUTOFLUSH)
      cout << unitbuf;
  }
} fast_ios_;

int main() {
  long k;
  cin >> k;
  if (~k & 1) {
    cout << -1 << endl;
    return 0;
  }
  long r = 7 % k;
  long ans = 1;
  while (r != 0) {
    r = (r * 10 + 7) % k;
    ans++;
    if (ans == 1e9) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << ans << endl;
  return 0;
}

Fixed submission:
#define LOCAL
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;
using ll = long long;
using P = pair<int, int>;

#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define ALL(x) ((x).begin(), (x).end())
#define SIZE(x) ((int)(x).size())
#define BITS(x) (1LL << (x))

constexpr int INF = 1001001001;
constexpr ll LINF = 1001001001001001001LL;
constexpr int MOD = 1000000007;
constexpr double EPS = 1e-10;
constexpr double PI = 3.141592653589793;
constexpr int dx[4] = {1, 0, -1, 0};
constexpr int dy[4] = {0, 1, 0, -1};
constexpr int dx8[8] = {1, 1, 1, 0, 0, -1, -1, -1};
constexpr int dy8[8] = {1, 0, -1, 1, -1, 1, 0, -1};

random_device seed_gen;
mt19937 mrand(seed_gen());
mt19937_64 mrand64(seed_gen());
int rand32(int x) { return mrand() % x; }
ll rand64(long x) { return mrand64() % x; }

template <typename A, typename B>
ostream &operator<<(ostream &out, const pair<A, B> &a) {
  out << "(" << a.first << "," << a.second << ")";
  return out;
}
template <typename T, size_t N>
ostream &operator<<(ostream &out, const array<T, N> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ",");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T> ostream &operator<<(ostream &out, const vector<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ",");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T, class Cmp>
ostream &operator<<(ostream &out, const set<T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ",");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream &operator<<(ostream &out, const map<U, T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &p : a) {
    out << (first ? "" : ",");
    out << p.first << ": " << p.second;
    first = 0;
  }
  out << "}";
  return out;
}
#ifdef LOCAL
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...) 42
#endif
template <typename Arg1> void __f(const char *name, Arg1 &&arg1) {
  cerr << name << ": " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char *names, Arg1 &&arg1, Args &&...args) {
  const char *comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << ": " << arg1 << " |";
  __f(comma + 1, args...);
}

struct fast_ios {
  static constexpr int IOS_PREC = 15;
  static constexpr bool AUTOFLUSH = false;

  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(IOS_PREC);
    if (AUTOFLUSH)
      cout << unitbuf;
  }
} fast_ios_;

int main() {
  long k;
  cin >> k;
  if (~k & 1) {
    cout << -1 << endl;
    return 0;
  }
  long r = 7 % k;
  long ans = 1;
  while (r != 0) {
    r = (r * 10 + 7) % k;
    ans++;
    if (ans == 1e8) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << ans << endl;
  return 0;
}