#include <iostream>

using namespace std;

bool gcd(long long a, long long b) {
    if (b == 0) {
        return a;
    }

    return gcd(b, a % b);
}

int main()
{
    long long a, b, m = -1;


    cin >> a >> b;

    for (long long i = b; i >= a; i--) {
        for (long long j = a; i - j > m; j++) {
            if (gcd(i, j) == 1) {
                m = i - j;
            }
        }
    }

    if (m == -1) {
        cout << "-1" << endl;
    } else {
        cout << m + 1 << endl;
    }

    return 0;
}