#include <bits/stdc++.h>

using namespace std;

int main() {
    long long a, b;
    cin >> a >> b;

    for(long long diff=b-a+1;diff>=0;diff--) {
        for(long long x=a, y=x+diff; y<=b; x++, y++) {
            if(__gcd(x, y) == 1) {
                cout << diff + 1;
                return 0;
            }
        }
    }

    cout << -1;
    return 0;
}