#include<iostream>
#include<vector>
#include<limits.h>
#include<math.h>

using namespace std;

void get_div_num(int v, vector<int>& a)
{
    for (int i = 2; i <= sqrt(v); ++i)
    {
        if (v % i == 0)
        {
            a.push_back(i);
            if (v / i != i)
            {
                a.push_back(v / i);
            }
        }
    }
}
int Jump(int n, int m)
{
    vector<int> res(m + 1, INT_MAX);
    res[n] = 0;
    for (int i = n; i < m; ++i)
    {
        if (res[i] == INT_MAX)
        {
            continue;
        }
        vector<int> a;
        get_div_num(i, a);
        for (int j = 0; j < a.size(); ++j)
        {
            if (a[j] + i <= m && res[a[j] + i] != INT_MAX)
            {
                res[a[j] + i] = min(res[a[j] + i], res[i] + 1);
            }
            else if (a[j] + i <= m)
            {
                res[a[j] + i] = res[i] + 1;
            }
        }
    }
    return res[m] == INT_MAX ? -1 : res[m];
}
int main()
{
    int n, m;
    int count = 0;
    while (cin >> n >> m)
    {
        count = Jump(n, m);
        cout << count << endl;
    }
    return 0;
}