Problem p02596 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;

//////////////////

#ifdef DEBUG
#include "debug.h"
#define dump(...)                                                              \
  DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                              \
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl  \
          << "    ",                                                           \
      dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

// http://beet-aizu.hatenablog.com/entry/2018/04/08/145516
template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

//////////////////

#define int long long

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)

#define YES puts("YES")
#define Yes puts("Yes")
#define NO puts("NO")
#define No puts("No")
#define ALL(v) (v).begin(), (v).end()
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

//*
#define mod                                                                    \
  1000000007 /*/                                                               \
#define mod 998244353 //*/

typedef pair<int, int> P;
#define INF (1LL << 60)

void solve() {
  int K;
  cin >> K;
  if (K % 2 == 0)
    cout << -1 << endl;
  else {
    int t = 0;
    for (int i = 1;; i++) {
      t = t * 10 + 7;
      t %= K;
      if (t == 0) {
        cout << i << endl;
        return;
      }
    }
  }
}

signed main() {
  cout << fixed << setprecision(18);
  cerr << fixed << setprecision(18);
  solve();
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

//////////////////

#ifdef DEBUG
#include "debug.h"
#define dump(...)                                                              \
  DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                              \
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl  \
          << "    ",                                                           \
      dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

// http://beet-aizu.hatenablog.com/entry/2018/04/08/145516
template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

//////////////////

#define int long long

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)

#define YES puts("YES")
#define Yes puts("Yes")
#define NO puts("NO")
#define No puts("No")
#define ALL(v) (v).begin(), (v).end()
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

//*
#define mod                                                                    \
  1000000007 /*/                                                               \
#define mod 998244353 //*/

typedef pair<int, int> P;
#define INF (1LL << 60)

void solve() {
  int K;
  cin >> K;
  if (K % 2 == 0)
    cout << -1 << endl;
  else {
    int t = 0;
    for (int i = 1;; i++) {
      t = t * 10 + 7;
      t %= K;
      if (t == 0) {
        cout << i << endl;
        return;
      }
      if (i > 100000000) {
        cout << -1 << endl;
        return;
      }
    }
  }
}

signed main() {
  cout << fixed << setprecision(18);
  cerr << fixed << setprecision(18);
  solve();
}