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 seven[1000001];
                int K,i;

                scanf("%d",&K);

                if(K % 2 == 0){
                        printf("-1");
                        return 0;
                }
                seven[1] = 7 % K;

                for(i = 2; i <= K; i++){
                        seven[i] = (seven[i-1] * 10 + 7) % K;
                        if(seven[i]==0)break;
                }

                for (i = 1;i <= K ; i++){
                        if(seven[i]==0){
                                printf("%d",i);
                                return 0;
                        }
                }
                /* if(i<=K)printf("%d",i); */
                /* else printf("-1"); */
                printf("-1");
                return 0;
}