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: Wrong Answer)

#include <stdio.h>
#define ll unsigned long long

void main(){
    unsigned long long K;
    unsigned long long ans = 7;
    int i = 0;
    scanf("%lld",&K);
    if(K%2 == 0){printf("-1\n");return;}
    unsigned long long npcp = 0;
    while(1){
        int f = npcp*10/K;
        npcp = (npcp*10)%(K);
        if(npcp == 0 && f > 0){printf("%d\n",i);return;}
        npcp = npcp + 7;
        i++;
        if(i >= __LONG_MAX__){break;}
    }
    printf("-1\n");
}