#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using p = pair<int, int>;
using P = pair<ll, ll>;
using V = vector<int>;
using VV = vector<vector<int>>;
using VVV = vector<vector<vector<int>>>;
using VL = vector<ll>;
using VVL = vector<vector<ll>>;
using VVVL = vector<vector<vector<ll>>>;
using Graph = VV;
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(a) (a).begin(),(a).end()
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; }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
int Inf = (1 << 30);
ll inf = (1LL << 60);
const int MOD = 1e9 + 7;
int main() {
	int a, b;
	cin >> a >> b;
	for (int i = 2; i <= max(a, b); i++) {
		if (a % i == 0 && b % i == 0) {
			cout << -1 << endl;
			return 0;
		}
	}
	vector<bool> dp(110000, false);
	dp[0] = true;
	rep(i, 100000) {
		if (dp[i]) {
			dp[i + a] = true;
			dp[i + b] = true;
		}
	}
	int ans = 0;
	rep(i, 100000) {
		if (!dp[i]) ans++;
	}
	cout << ans << endl;
	return 0;
}