#include <iostream>
#define LL long long
using namespace std;

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

LL A,B;

int main(){
    cin >> A >> B;
    if (A==B){
        cout << -1 << "\n";
        return 0;
    }

    LL i,j,dif=0;
    for (i=A; i<=min(B,A+2000); i++)
        for (j=B; j>min(i,B-2000); j--)
            if (gcd(i,j)==1) dif=max(dif,j-i+1);

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