#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {
  ll a,b,c=1,cnt=1;
  cin>>a>>b;
  if(b==0){
    cout<<1<<endl;
    return 0;
  }
  for(int i=a; a<10; a++){
    c *= i;
    cnt++;
    if(c==-1){
      cout<<cnt<<endl;
      return 0;
    }
  }
  cout<<-1<<endl;
}