#include<iostream>
#include<vector>
#include<algorithm>
#include<cstring>
#include<cassert>
#include<cmath>
#include<ctime>
#include<iomanip>
#include<numeric>
#include<stack>
#include<queue>
#include<map>
#include<unordered_map>
#include<set>
#include<unordered_set>
#include<bitset>
#include<random>
#include<functional>
#include<utility>
using namespace std;
int gcd(int a,int b)
{
	while(b)
	{
		a %= b;
		swap(a,b);
	}
	return a;
}
int A,B;
void solve()
{
	cin >> A >> B;
	int G = gcd(A,B);
	if(G > 1) cout << -1 << endl;
	else
	{
		int ans = 0;
		for(int i = 0;i < A;i++) ans += i*B/A;
		cout << ans << endl;
	}
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt = 1;
	/* cin >> tt; */
	while(tt--) solve();
}