#include <bits/stdc++.h>
using namespace std;

#define ll long long
const int K = 2000;

ll gcd(ll a, ll b) {
    if(!b)
        return a;
    return gcd(b, a % b);
}

int main() {
    ll a, b; cin >> a >> b;
    ll ans = -2;

    for(ll i = a; i <= min(b, a + K); ++i)
        for(ll j = b; j >= max(a, b - K); --j)
            if(gcd(i, j) == 1)
                ans = max(ans, max(i, j) - min(i, j));
    
    cout << ans + 1 << "\n";
}