#include <iostream>
#define ull unsigned long long
using namespace std;

ull gcd(ull a, ull b) {
    if(b == 0) return a;
    return gcd(b, a%b);
}

int main() {
    ull a, b, result = -1;
    cin>>a>>b;

    ull gap = 1000;
    for(ull i=a; i<=min(a+gap, b); ++i)
        for(ull j=b; j>=max(a, b-gap); --j)
            if(i <= j && gcd(i, j) == 1)
                result = max(result, j-i+1);

    cout<<result<<"\n";
    return 0;
}