#include <iostream>
#include <algorithm>

using namespace std;

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