Problem p02596 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
// #include<ext/pb_ds/assoc_container.hpp>
// #include<ext/pb_ds/tree_policy.hpp>
//  using namespace __gnu_pbds;
using namespace std;
#define ll long long
#define int long long // comment it out when MLE
#define endl "\n"
// template <typename T>
// using ordered_set = tree<T, null_type, less<int>, rb_tree_tag,
// tree_order_statistics_node_update>;

#define gok                                                                    \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);
// # define m 163577857
#define mo 1000000007
#define pi acos(-1)
#define fi first
#define se second
#define pb push_back
#define pii pair<int, int>
#define REP(i, n) for (int i = 1; i <= n; i++)
const ll mod = 1e9 + 7;
// const int K = 2;
// typedef vector<vector<int> > matrix;
/*
          int add(int a, int b,int mod) {
                a%=mod;
                b%=mod;
                a += b;
                if (a >= mod) a -= mod;
                if(a<0)
                {
                    a += mod;
                    a %= mod;
                }
                return a;
            }

            int mul(ll a, int b,int mod) {
                a%=mod;
                b%=mod;
                a *= b;
                a += mod;
                return a % mod;
            }
            int po(int a,int b,int mod)
            {
                if(b==0)
                    return 1;
                if(b%2==0)
                    return po(mul(a,a,mod),b/2,mod);
                return mul(a,po(mul(a,a,mod),(b-1)/2,mod),mod);
            }


            ll fact[200003];
            ll inv[200003];

            void fact0()
            {
                int i,j;
                fact[0]=1;
                for(i=1;i<=200000;i++)
                {
                    fact[i]=i*fact[i-1]%mod;
                }
                inv[0]=1;
                inv[1]=1;
                ll p=mod;
                for (i=2; i<=200000; i++)
                    inv[i] = (p - (p/i) * inv[p%i] % p) % p;

                for(i=2;i<=200000;i++)
                {
                    inv[i]*=inv[i-1];
                    inv[i]%=mod;
                }
            }
            int help(int a,int b)
            {
               // cout<<a<<" "<<b<<endl;
                int an = fact[a];
                //cout<<an<<endl;
                an *= inv[b];
                an %= mod;
                an *= inv[a-b];
                an %= mod;
                return an;
            }
/*
          void random()
            {
              srand(time(0));
            for (int i = 0; i < n; i ++)
                perm[i] = i + 1;
            random_shuffle(perm, perm + n);
            }
int ncr(int a,int b)
{
    int tt = fact[a];
    tt %= mod;
    tt *= inv[a-b];
    tt %= mod;
    tt *= inv[b];
    tt %= mod;
    return tt;
}
*/

signed main() {
  gok

      int k;
  cin >> k;

  if (k % 2 == 0)
    cout << -1;
  else {
    int st = 7;
    st %= k;
    int cc = 1;
    while (st || cc > 1e7) {
      st = 10 * st + 7;
      st %= k;
      cc++;
    }
    if (cc > 1e7)
      cc = -1;
    cout << cc;
  }

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
// #include<ext/pb_ds/assoc_container.hpp>
// #include<ext/pb_ds/tree_policy.hpp>
//  using namespace __gnu_pbds;
using namespace std;
#define ll long long
#define int long long // comment it out when MLE
#define endl "\n"
// template <typename T>
// using ordered_set = tree<T, null_type, less<int>, rb_tree_tag,
// tree_order_statistics_node_update>;

#define gok                                                                    \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);                                                                  \
  cout.tie(0);
// # define m 163577857
#define mo 1000000007
#define pi acos(-1)
#define fi first
#define se second
#define pb push_back
#define pii pair<int, int>
#define REP(i, n) for (int i = 1; i <= n; i++)
const ll mod = 1e9 + 7;
// const int K = 2;
// typedef vector<vector<int> > matrix;
/*
          int add(int a, int b,int mod) {
                a%=mod;
                b%=mod;
                a += b;
                if (a >= mod) a -= mod;
                if(a<0)
                {
                    a += mod;
                    a %= mod;
                }
                return a;
            }

            int mul(ll a, int b,int mod) {
                a%=mod;
                b%=mod;
                a *= b;
                a += mod;
                return a % mod;
            }
            int po(int a,int b,int mod)
            {
                if(b==0)
                    return 1;
                if(b%2==0)
                    return po(mul(a,a,mod),b/2,mod);
                return mul(a,po(mul(a,a,mod),(b-1)/2,mod),mod);
            }


            ll fact[200003];
            ll inv[200003];

            void fact0()
            {
                int i,j;
                fact[0]=1;
                for(i=1;i<=200000;i++)
                {
                    fact[i]=i*fact[i-1]%mod;
                }
                inv[0]=1;
                inv[1]=1;
                ll p=mod;
                for (i=2; i<=200000; i++)
                    inv[i] = (p - (p/i) * inv[p%i] % p) % p;

                for(i=2;i<=200000;i++)
                {
                    inv[i]*=inv[i-1];
                    inv[i]%=mod;
                }
            }
            int help(int a,int b)
            {
               // cout<<a<<" "<<b<<endl;
                int an = fact[a];
                //cout<<an<<endl;
                an *= inv[b];
                an %= mod;
                an *= inv[a-b];
                an %= mod;
                return an;
            }
/*
          void random()
            {
              srand(time(0));
            for (int i = 0; i < n; i ++)
                perm[i] = i + 1;
            random_shuffle(perm, perm + n);
            }
int ncr(int a,int b)
{
    int tt = fact[a];
    tt %= mod;
    tt *= inv[a-b];
    tt %= mod;
    tt *= inv[b];
    tt %= mod;
    return tt;
}
*/

signed main() {
  gok

      int k;
  cin >> k;

  if (k % 2 == 0)
    cout << -1;
  else {
    int st = 7;
    st %= k;
    int cc = 1;
    while (st && cc <= 1e7) {
      st = 10 * st + 7;
      st %= k;
      cc++;
    }
    if (cc > 1e7)
      cc = -1;
    cout << cc;
  }

  return 0;
}