#include<bits/stdc++.h>
using namespace std;
const unsigned long long N=0x3f;
unsigned long long a[N];
int main()
{
	unsigned long long n,k;
	cin>>n>>k;
	unsigned long long sum=1;
	for(int i=0;i<k;i++)
	{
		sum=sum*10;
	}
	unsigned long long ans=0;
	while(sum--)
	{
		n=n*k;
		a[ans]=n%10;
		if(ans>=1)
		{
			unsigned long long d=a[ans];
			if(d==a[0])
			{
				cout<<ans;
				return 0;
			}
		}
		ans++;
	}
	cout<<-1;
	return 0;
}

本蒟蒻样例都对