#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int(i) = 0; (i) < (n); (i)++)
#define FOR(i, m, n) for(int(i) = (m); (i) < (n); (i)++)
#define All(v) (v).begin(), (v).end()
#define pb push_back
#define MP(a, b) make_pair((a), (b))
template <class T> vector<T> make_vec(size_t a, T val) {
    return vector<T>(a, val);
}
template <class... Ts> auto make_vec(size_t a, Ts... ts) {
    return vector<decltype(make_vec(ts...))>(a, make_vec(ts...));
}
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using Graph = vector<vector<int>>;
template <typename T> struct edge {
    int to;
    T cost;
    edge(int t, T c) : to(t), cost(c) {}
};
template <typename T> using WGraph = vector<vector<edge<T>>>;
const int INF = 1 << 30;
const ll LINF = 1LL << 60;
const int MOD = 1e9 + 7;

int main() {
    int A, B;
    cin >> A >> B;
    if(__gcd(A, B) != 1) {
        cout << -1 << endl;
        return 0;
    }
    vector<bool> ok(2 * A * B + 1);
    for(int i = 0; i <= B; i++) {
        for(int j = 0; j <= A; j++) {
            ok[A * i + B * j] = 1;
        }
    }
    int res = 0;
    for(int i = 1; i <= A * B; i++) {
        if(!ok[i])
            res++;
    }
    cout << res << endl;
}