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: Accepted)

#include<bits/stdc++.h>
using namespace std;
int k,st[1100000],f[1100000];
int main() {
	scanf("%d",&k);
	st[1]=7%k;int r=1;
	if (st[1]==0) printf("1\n");
	else {
		f[st[1]]=1;
		while (r<=k) {
			int tmp=(st[r]*10+7)%k;
			st[++r]=tmp;
			if (f[st[r]]){
				printf("-1\n");
				return 0;
			}
			if (st[r]==0) {
				printf("%d\n",r);
				return 0;
			}
			f[st[r]]=1;
		}
	}
	return 0;
}