#include <iostream>
#include <algorithm>

using namespace std;

unsigned long long gcd(unsigned long long a, unsigned long long 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 = a; i <= b; ++i) {
        for (unsigned long long j = b; j >= a; --j) {
            if (gcd(i, j) == 1) {
                cout << j - i + 1;
                return 0;
            }
        }
    }
    cout << -1;
    return 0;
}