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 <stdio.h>

int main(){
    int k;
    int a[1000001];

    scanf("%d", &k);

    a[1] = 7 % k;
    for(int i = 2; i <= k; ++i){
        a[i] = (10 * a[i-1] + 7) % k;
    }

    int flag = 0;
    for(int i = 1; i <= k; ++i){
        if(a[i] == 0){
            printf("%d\n", i);
            flag = 1;
            break;
        }
    }
    if(flag == 0) printf("-1\n");

    return 0;
}