#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <cstring>
#include <string>
#include <vector>
#include <set>
#include <map>
using namespace std;

const long long MAGIC = 2000;
long long gcd(long long a, long long b) {
    while(b) {
        a %= b;
        swap(a, b);
    }
    return a;
}

int main() {
    long long a, b;
    cin >> a >> b;
    long long ans = -1;
    for(long long i = a; i <= b && i < a + MAGIC; ++i) {
        for(long long j = max(i, b - MAGIC); j <= b; ++j) {
            if(gcd(i, j) == 1) {
                ans = max(ans, j-i+1);
            }
        }
    }
    cout << ans << endl;
    
    
    
    return 0;
}