#include <iostream>
using namespace std;

int gcd(int a, int b) {
    return !b ? a : gcd(b, a % b);
}

int main() {
    int a, b;
    const int MAX_D = 3000;
    int best = -1;
    
    cin >> a >> b;
    
    for (int i = a; i <= a + MAX_D && i <= b; ++i) {
        for (int j = b; j >= b - MAX_D && j >= a && j >= i && j - i + 1 >= best; --j)
            if (gcd(i, j) == 1 && j - i + 1 > best) {
                best = j - i + 1;
            }
    }
    
    cout << best << "\n";
}