#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <math.h>
#include <complex>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <bitset>
#include <functional>
#include <assert.h>
#define int long
#define ll long long
#define rng(i,a,b) for(int i=int(a);i<int(b);i++)
#define rep(i,b) rng(i,0,b)
#define gnr(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define per(i,b) gnr(i,0,b)
#define eb emplace_back
#define all(x) x.begin(),x.end()
#define SZ(x) x.size()
#define fi first
#define se second
const int INF = 1e9;
using namespace std;

template<class t> using vc=vector<t>;
template<class t> using vvc=vc<vc<t>>;

using pi=pair<int,int>;
using vi=vc<int>;

template<class T>
inline bool chmax(T &a,T b){if(a<b){a=b;return true;}return false;}
template<class T>
inline bool chmin(T &a,T b){if(a>b){a=b;return true;}return false;}

int gcd(int a,int b){return b ? gcd(b,a%b) : a;}
int lcm(int a,int b){return a/gcd(a,b)*b;}

int m[100000];
signed main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout<<fixed<<setprecision(20);

    int A,B; cin >> A >> B;
    if(gcd(A,B)>=2) cout << -1 << endl;
    else {
        int cnt=0;
        
        rep(j,lcm(A,B)/A+1){
            rep(k,lcm(A,B)/B+1){
                m[j*A+k*B]++;
            }
        }
        
        rng(i,1,lcm(A,B)+1){
            if(m[i]==0) cnt++;
        }
        cout << cnt << endl;
    }
}