#include <iostream>

using namespace std;

int64_t Gcd(int64_t x, int64_t y) {
   return y ? Gcd(y, x % y) : x;
}

int main() {
   int64_t A, B;
   cin >> A >> B;

   for (int64_t i = B - A; i > 0; --i)
      for (int64_t j = A; j <= B - i; ++j)
         if (Gcd(j, j + i) == 1) {
            cout << i + 1 << "\n";
            return 0;
         }

   cout << "-1\n";
   return 0;
}