#include <bits/stdc++.h>
using namespace std;
 
using ll = long long;
using ld = long double;
#define FOR(i, m, n) for (int i = (m); i < (n); i++)
#define FORR(i, m, n) for (int i = (m); i >= (n); i--)
#define REP(i, n) FOR(i, 0, (n))
#define REPR(i, n) FORR(i, (n) - 1, 0)
#define REP1(i, n) FOR(i, 1, (n) + 1)
#define REPS(c, s) for (char c : s)
#define ALL(c) (c).begin(), (c).end()
#define SORT(c) sort(ALL(c))
#define REV(c) reverse(ALL(c))
#define sz(v) (int)v.size()
#define endl '\n'
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;}
const int MOD = 1000000007;
const int INF = 1000000001;
const ll LINF = 1000000001000000001LL;
 
void solve();
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(numeric_limits<double>::max_digits10);
    solve();
    return 0;
}

void solve() {
    int a, b;
    cin >> a >> b;
    if (a % 2 == 0 && b % 2 == 0) {
        cout << -1 << endl;
        return;
    }
    if (a > b) swap(a, b);
    bool ok = true;
    REP1(i, b - 1) {
        bool k = false;
        REP1(j, b) {
            if ((a * j) % b == i) {
                k = true;
                break;
            }
        }
        if (!k) {
            ok = false;
            break;
        }
    }
    if (!ok) {
        cout << - 1 << endl;
        return;
    }
    set<int> s;
    REP(i, 2000) REP(j, 2000) {
        s.emplace(a * i + b * j);
    }
    int ans = 0;
    REP1(i, *(prev(s.end())) * 2 / 3) {
        if (s.count(i) == 0) ans++;
    }
    // cout << a * 999 + b * 999 << endl;
    // cout << *(prev(s.end())) << endl;
    cout << ans << endl;
}