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>
#include<math.h>
long long int f(long long int x)
{
    int count = 0;
    while(x > 0)
    {
        x = x / 10;
        count++;
    }
    return count;
}
int main(void)
{
    long long int n,i,j,mod = 1000000000 + 7,sum = 0,x[200001],y[200001],d,count = 0,k,a,nana = 0;
    
    scanf("%lld",&k);
    if(k % 2 == 0 || k % 5 == 0)
    {
        printf("-1\n");
    }
    else
    {
        a = f(k);
        for(i = 0 ; i < a - 1; i++)
        {
            nana = nana * 10 + 7;
        }
        count = a - 1;
        while(1)
        {
            if(nana % k == 0)
            {
                printf("%lld\n",count);
                return 0;
            }
            j = nana % k;
            nana = j * 10 + 7;
            count++;
        }
    }
    
    return 0;
}