#include <iostream>
#define dist 1000

using namespace std;

long long i, j, a, b, ans = -1, ca = 0, cb = 0;

long long cmmdc(long long a, long long b)
{
    if (b == 0)
        return a;
    return cmmdc(b, a % b);
}

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

    for (i = a, ca = 0; i <= b && ca <= dist; ++ i, ++ ca)
        for (j = b, cb = 0; j >= i && cb <= dist; -- j, ++ cb)
           // cout << i << " " << j << " " << ans << '\n';
        if (cmmdc(i, j) == 1)
            ans = max(ans, j - i + 1);

    cout << ans;
    return 0;
}