#include <cstdio>

using namespace std;

const int SPAN = 10000;

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

int main() {
    //freopen("file.in", "r", stdin);
    //freopen("file.out", "w", stdout);

    long long a, b;
    register int i, j;

    scanf("%lld %lld", &a, &b);
    for(i = 0; i <= SPAN; i++) {
        for(j = 0; j <= SPAN; j++) {
            if(gcd(a + i, b - j) == 1) {
                printf("%lld\n", b - j - a - i + 1);
                return 0;
            }
        }
    }
    printf("-1\n");
    return 0;
}