#include <bits/stdc++.h>

using namespace std;

int main()
{
    long long A, B;
    int it = 0;

    cin >> A >> B;

    long long sol = -1;

    while (A <= B && it <= 2)
    {
        if (__gcd(A, B) == 1)
            sol = max(sol, B - A + 1);

        A++;
        it++;
    }

    cout << sol << "\n";

    return 0;
}