#include <bits/stdc++.h>

using namespace std;

long long gcd(long long A, long long B) {
    if (B == 0)
        return A;
    return gcd(B, A % B);
}

int main() {
    long long a, b;
    long long res = -1;
    cin >> a >> b;
    for (long long my_a = a; my_a <= a + 1000 && my_a <= b; ++my_a)
        for (long long my_b = b; my_b >= b - 1000 && my_b >= a; --my_b) {
            long long gc = gcd(my_a, my_b);
            if (gc == 1 && my_b - my_a + 1 > res)
                res = my_b - my_a + 1;
        }
    cout << res;
    return 0;
}