#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
#define cinf(n,x) for(int i=0;i<(n);i++)cin>>x[i];
#define ft first
#define sc second
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(),(v).end()
#define LB(a,x) lb(all(a),x)-a.begin()
#define UB(a,x) ub(all(a),x)-a.begin()
#define mod 1000000007
//#define mod 998244353
#define FS fixed<<setprecision(15)
using namespace std;
typedef long long ll;
const double pi=3.141592653589793;
template<class T> using V=vector<T>;
using Graph = vector<vector<int>>;
using P=pair<ll,ll>;
typedef unsigned long long ull;
typedef long double ldouble;
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline void out(T a){ cout << a << '\n'; }
void YN(bool ok){if(ok) cout << "Yes" << endl; else cout << "No" << endl;}
//void YN(bool ok){if(ok) cout << "YES" << endl; else cout << "NO" << endl;}


const ll INF=1e18;
const int mx=200005;

bool dp[1000001];

int main(){
    cin.tie(0);ios::sync_with_stdio(false);
    ll a,b;
    cin>>a>>b;
    if(__gcd(a,b)!=1) cout<<-1<<endl;
    else{
        ll p=1000000;
        dp[0]=1;
        for(int i=0;i<=1000000;i++){
            if(dp[i]){
                if(i+a<=p) dp[i+a]=1;
                if(i+b<=p) dp[i+b]=1;
            }
            
        }
        int ans=0;
        rep(i,p+1) if(!dp[i]) ans++;
        out(ans);
    }
}