Problem p02596 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string.h>
#include <unordered_map>
#include <vector>

using namespace std;

#define COUT(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl

template <class T> void printvec(const vector<T> &v) {
  for (auto x : v) {
    cout << x << " ";
  }
  cout << endl;
}
template <class T> void printtree(const vector<vector<T>> &tree) {
  for (long long i = 0; i < tree.size(); ++i) {
    cout << i + 1 << ": ";
    printvec(tree[i]);
  }
}
template <class T, class U> void printmap(const map<T, U> &mp) {
  for (auto x : mp) {
    cout << x.first << "=>" << x.second << endl;
  }
}

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;
}

#define rep(i, n) for (ll i = 0; i < n; ++i)
#define all(s) s.begin(), s.end()
#define sz(x) (ll)(x).size()
#define fr first
#define sc second
#define mp make_pair
#define pb push_back
#define eb emplace_back

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> triple;
typedef double D;
typedef vector<ll> vl;
typedef vector<P> vp;

const ll INF = 1e9;
ll MOD;

// Mod int
// cf. https://www.youtube.com/watch?v=1Z6ofKN03_Y
struct mint {
  ll x;
  mint(ll x = 0) : x((x + MOD) % MOD) {}
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint &operator-=(const mint a) {
    if ((x += MOD - a.x) >= MOD)
      x %= MOD;
    return *this;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t) {
      return 1;
    }
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // ref. https://qiita.com/drken/items/3b4fdf0a78e7a138cd9a
  mint invWithGcd() const {
    ll a = x, b = MOD, u = 1, v = 0;
    while (b) {
      ll t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    u %= MOD;
    if (u < 0) {
      u += MOD;
    }
    return mint(u);
  }

  // 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, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

// int main(int argc, char** argv) {
//   // int p;
//   // cin >> p;
//
//   MOD = 13;
//   mint p(10);
//   cout << (p + 15).x << endl;    // 12 (25 % 13)
//   cout << (p - 15).x << endl;    // 8  (-5 % 13)
//   cout << (p * 2).x << endl;     // 7  (20 % 13)
//   cout << (p.pow(3)).x << endl;  // 12 (1000 % 13)
//   cout << (p / 3).x << endl;     // 12 (12 * 3 = 10 (36 % 13))
//   cout << p * mint(3).invWithGcd().x << endl; // 12 (12 * 3 = 10 (36 % 13))
//
//   mint p2(-3);
//   cout << p2.x << endl; // 10 (-3 % 13)
// }
int main(int argc, char **argv) {
  cin.tie(NULL);
  cout.tie(NULL);
  ios_base::sync_with_stdio(false);
  // cout << setprecision(10) << fixed;

  ll k;
  cin >> k;
  MOD = k;
  if (k % 2 == 0) { // even
    cout << -1 << endl;
    return 0;
  }

  mint now = 0;
  rep(i, k * 10) {
    now += mint(10).pow(i) * 7;
    if (now.x == 0) {
      cout << i + 1 << endl;
      return 0;
    }
  }

  cout << -1 << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string.h>
#include <unordered_map>
#include <vector>

using namespace std;

#define COUT(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl

template <class T> void printvec(const vector<T> &v) {
  for (auto x : v) {
    cout << x << " ";
  }
  cout << endl;
}
template <class T> void printtree(const vector<vector<T>> &tree) {
  for (long long i = 0; i < tree.size(); ++i) {
    cout << i + 1 << ": ";
    printvec(tree[i]);
  }
}
template <class T, class U> void printmap(const map<T, U> &mp) {
  for (auto x : mp) {
    cout << x.first << "=>" << x.second << endl;
  }
}

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;
}

#define rep(i, n) for (ll i = 0; i < n; ++i)
#define all(s) s.begin(), s.end()
#define sz(x) (ll)(x).size()
#define fr first
#define sc second
#define mp make_pair
#define pb push_back
#define eb emplace_back

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> triple;
typedef double D;
typedef vector<ll> vl;
typedef vector<P> vp;

const ll INF = 1e9;
ll MOD;

// Mod int
// cf. https://www.youtube.com/watch?v=1Z6ofKN03_Y
struct mint {
  ll x;
  mint(ll x = 0) : x((x + MOD) % MOD) {}
  mint &operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint &operator-=(const mint a) {
    if ((x += MOD - a.x) >= MOD)
      x %= MOD;
    return *this;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const {
    if (!t) {
      return 1;
    }
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // ref. https://qiita.com/drken/items/3b4fdf0a78e7a138cd9a
  mint invWithGcd() const {
    ll a = x, b = MOD, u = 1, v = 0;
    while (b) {
      ll t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    u %= MOD;
    if (u < 0) {
      u += MOD;
    }
    return mint(u);
  }

  // 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, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

// int main(int argc, char** argv) {
//   // int p;
//   // cin >> p;
//
//   MOD = 13;
//   mint p(10);
//   cout << (p + 15).x << endl;    // 12 (25 % 13)
//   cout << (p - 15).x << endl;    // 8  (-5 % 13)
//   cout << (p * 2).x << endl;     // 7  (20 % 13)
//   cout << (p.pow(3)).x << endl;  // 12 (1000 % 13)
//   cout << (p / 3).x << endl;     // 12 (12 * 3 = 10 (36 % 13))
//   cout << p * mint(3).invWithGcd().x << endl; // 12 (12 * 3 = 10 (36 % 13))
//
//   mint p2(-3);
//   cout << p2.x << endl; // 10 (-3 % 13)
// }
int main(int argc, char **argv) {
  cin.tie(NULL);
  cout.tie(NULL);
  ios_base::sync_with_stdio(false);
  // cout << setprecision(10) << fixed;

  ll k;
  cin >> k;
  MOD = k;
  if (k % 2 == 0) { // even
    cout << -1 << endl;
    return 0;
  }

  mint now = 0;
  rep(i, k + 5) {
    now += mint(10).pow(i) * 7;
    if (now.x == 0) {
      cout << i + 1 << endl;
      return 0;
    }
  }

  cout << -1 << endl;
  return 0;
}