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)

//https://atcoder.jp/contests/abc174/tasks/abc174_c
#include <stdio.h>
#include <math.h>
#include <stdlib.h>

int main() {

    int K= 0;
    int n= 0;
    
    printf("\n");
    scanf("%d", &K);
    
    int *B= malloc(sizeof(int)*K*2);

    B[0]= 7;
    
    if(K%2==0){
        printf("%d", -1);
    }
    else if(K==1||K==7){
        printf("%d", 1);
    }
    else{
        while(1){
            B[n]= 10*B[n-1] + 7;
            //B[n]= 7/9*(pow(10, n)-1);
            if(B[n]%K==0){
                printf("%d", n+1);
                break;
            }
            else if(n==1999999){
                break;
            }
            else{
               n++;
            }
        }
        
    }
    
    
    return 0;
}