#include<bits/stdc++.h>
#define in cin
#define out cout
#define int64 unsigned long long
using namespace std;

ifstream f("date.in");

int64 a;
int64 b;


int64 gcd(int64 a, int64 b) {
    if(b != 0) {
        return gcd(b, a % b);
    }

    return a;

}
void read() {
    in >> a;
    in >> b;
}


void solve() {

    if(a == 1) {
        out << b - a + 1;
        return;
    }

    int64 right = b;

    while(right >= a && gcd(a, right) != 1) {
        right--;
    }

    if(right < a) {
        out << -1;
    } else {
        int64 length = right - a + 1;
        int64 maxx = 0;

        for(int64 i = a; i + length <= b; i++) {
            for(int64 j = a + length - 1; j <= b; j += (length - 1)) {
                if(gcd(i, j) == 1) {
                    maxx = max(maxx, j - i + 1);
                }
            }
        }
        if(maxx == 0) {
            out << -1;
        } else out << maxx;
    }


}

int main() {
    read();
    solve();
    return 0;
}