#include <iostream>

using namespace std;

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

int main()
{
    int a, b;
    bool w = 0;

    cin >> a >> b;

    if(cmmdc(a, b) == 1)
    {
        cout << b-a+1;
        w = 1;
    }
    else
        for(int i = 1; i < b-a; ++i)
        {
            if(cmmdc(a, b-i) == 1)
            {
                cout << b-i-a+1;
                i = b+1;
                w = 1;
            }
            else if(cmmdc(a+i, b) == 1)
            {
                cout << b-a-i+1;
                i = b+1;
                w = 1;
            }
        }
    if(!w)
        cout << -1;
    return 0;
}