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: Runtime Error)

/**
*    author:  souzai32
*    created: 12.08.2020 15:37:08
**/

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using namespace std;

int main() {

    int n,k;
    cin >> n >> k;
    vector<int> a(n);
    rep(i,n) cin >> a.at(i);
    sort(a.begin(),a.end());

    int front=1,back=n;
    int l=a.at(n-1)/2,count;
    int ans=a.at(n-1);

    while(1){
        count=0;
        rep(i,n) count+=(a.at(i)-1)/l;
        if(count<=k){
            ans=l;
            back=l;
        }
        else front=l+1;
        l=(front+back)/2;
        if(front>=back) break;
    }
    
    cout << ans << endl;

    return 0;
}