#include <iostream>
#include <algorithm>

using namespace std;

unsigned long long gcd(int a, int b) {
    if (b == 0)
        return a;
    return gcd(b, a % b);
}

int main()
{
    ios::sync_with_stdio(false);
    unsigned long long a, b;
    cin >> a >> b;
    if (a == b) {
        cout << -1;
        return 0;
    }
    for (unsigned long long i = 0; i <= (b - a) / 2; ++i) {
        if (gcd(a + i, b - i) == 1) {
            cout << b - i - a - i + 1;
            return 0;
        }
    }
    cout << -1;
    return 0;
}