#include <iostream>
#include <vector>

const int primes[26] = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101};

bool check(int a, int b) {
    if (a - b < 0) {
        return false;
    }
    return true;
}

int compute(int x, int y) {
    int x_primes[20];
    int primes_nr = 0;
    int sum = y - x;
    std::vector<int> dp(y, INT32_MAX);
    dp[0] = 1;

    for (int i = 0; i < 26; i++) {
        if (x <= primes[i]) {
            break;
        }

        if (x % primes[i] == 0) {
            x_primes[primes_nr++] = primes[i];
            dp[primes[i]] = 1;
        }
    }

    for (int i = 1; i <= sum; i++) {
        for (int j = 0; j < primes_nr; j++) {
            if (check(i, x_primes[j]) && dp[i - x_primes[j]] != INT32_MAX) {
                dp[i] = std::min(dp[i], 1 + dp[i - x_primes[j]]);
            }
        }
    }

    if (dp[sum] == INT32_MAX) {
        return -1;
    }
    return dp[sum];
}

int main() {
    int x, y;

    while (1) {
        std::cin >> x >> y;
        if (x == 0 && y == 0) {
            break;
        }

        std::cout << compute(x, y) << '\n';
    }
}