//↓template↓

#include "bits/stdc++.h"
using namespace std;
#define Would
#define you
#define all(n)  n.begin(),n.end()
const long long INF = 1e18;
const long long MOD = 1e9 + 7;
const double pi = acos(-1);
const int SIZE = 1 << 17;
int dx[] = { 1,0 }, dy[] = { 0,1 }, alp[30];
long long fac[200005], finv[200005], inv[200005];
vector<long long>dij;
struct edge { long long to, cost; };
vector<vector<edge> >G;

long long mod_pow(long long a, long long b) {
	long long res = 1;
	while (b) {
		if (b & 1) { res = res * a%MOD; }
		a = a * a%MOD;
		b >>= 1;
	}
	return res;
}

void addedge(int from, int to, int cost) {
	G[from].push_back({ to,cost });
	G[to].push_back({ from,cost });
}

//↑template↑

int main() {
	long long a, b, ans = -1;
	cin >> a >> b;
	if (2 * a == b) { ans = a * 3; }
	else if (2 * b == a) { ans = b * 3; }
	else if (3 * b == a) { ans = 2 * b; }
	else if (a * 3 == b) { ans = 2 * a; }
	else if (a % 2 == 0 && a / 2 * 3 == b) { ans = a / 2; }
	else if (b % 2 == 0 && b / 2 * 3 == a) { ans = b / 2; }

	cout << ans << endl;
}