Problem:
Score : 300 points

Problem Statement

Takahashi loves the number 7 and multiples of K.

Where is the first occurrence of a multiple of K in the sequence 7,77,777,\ldots? (Also see Output and Sample Input/Output below.)

If the sequence contains no multiples of K, print -1 instead.

Constraints

1 \leq K \leq 10^6

K is an integer.

Input

Input is given from Standard Input in the following format:

K

Output

Print an integer representing the position of the first occurrence of a multiple of K. (For example, if the first occurrence is the fourth element of the sequence, print 4.)

Sample Input 1

101

Sample Output 1

4

None of 7, 77, and 777 is a multiple of 101, but 7777 is.

Sample Input 2

2

Sample Output 2

-1

All elements in the sequence are odd numbers; there are no multiples of 2.

Sample Input 3

999983

Sample Output 3

999982

Solution in C++ (GCC 9.2.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>

using namespace std;

int main()
{
    int k;
    cin >> k;
    vector<int> v;
    bool flag = true;
    int c = 1;
    int a = 7;
    a %= k;
    while (flag)
    {
        if (a == 0)
        {
            cout << c << endl;
            break;
        }
        else
        {
            a = a * 10 + 7;
            a %= k;
            c++;
            if (find(v.begin(), v.end(), a) != v.end())
            {
                v.push_back(a);
            }
            else
            {
                cout << -1 << endl;
                break;
            }
        }
    }
    return 0;
}