#include <iostream>
#include <cstdlib>
using namespace std;

unsigned long long a, b;

unsigned long long cmmdc(unsigned long long a, unsigned long long b) {
    if ( !b )
        return a;
    return cmmdc(b, a % b);
}

int main() {

    cin >> a >> b;

    if ( a == b ) {
        cout << -1;
        exit(0);
    }

    for ( unsigned long long i = a; i <= b; ++i )
        for ( unsigned long long j = b; j >= a; --j )
            if ( cmmdc(i,j) == 1 ) {
                cout << j - i + 1;
                exit(0);
            }

    cout << -1;




    return 0;
}