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)

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char *seven_cur, *seven;
int incrementSeven(int);

int main()
{
    int k, count = 1;
    char *temp;
    scanf("%d", &k);
    if (k % 2 == 0){
        printf("-1");
        return 0;
    }
    
    
    seven_cur = (char *)malloc(sizeof(char));
    seven = (char *)malloc(sizeof(char));
    strcpy(seven, "7");
    strcpy(seven_cur, seven);
    
    
    while(1){
        long long sevenL = strtoll(seven_cur, &temp, 10);
        //printf("%lld\n", sevenL);
        if (sevenL % k == 0){
            printf("%d", count);
            break;   
        }
        incrementSeven(++count);
    }
}

int incrementSeven(int length){
    char *buffer = (char *)realloc(seven_cur, length * sizeof(char));
    seven_cur = buffer;
    strcat(seven_cur, seven);
    //printf("%s\n", seven_cur);
    return 0;
}