#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>

using namespace std;

#define int long long
#define endl "\n"

constexpr long long INF = (long long)1e18;
constexpr long long MOD = 1'000'000'007; 

string yn(bool f){return f?"Yes":"No";}
string YN(bool f){return f?"YES":"NO";}



signed main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	cout<<fixed<<setprecision(10);
	
	const int MAX = 110000;
	
	int A, B, con = 0, LCM;
	vector<int> hoge(MAX);
	
	cin>>A>>B;
	
	// LCM = A/__gcd(A,B) * B;
	
	hoge[0] = true;
	
	for(int i = 0; i < MAX; i++){
		if(hoge[i] == true && i + A < MAX) hoge[i + A] = true;
	}
	for(int i = 0; i < MAX; i++){
		if(hoge[i] == true && i + B < MAX) hoge[i + B] = true;
	}
	for(int i = 1; i < MAX; i++){
		if(!hoge[i]) con++;
	}
	
	// for(int i = 0; i < 500; i++){
		// cout<<"i = "<<i<<" "<<hoge[i]<<endl;
	// }
	
	if(__gcd(A,B) == 1)cout<<con<<endl;
	else cout<<-1<<endl;
	
	return 0;
}