#include <string>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <climits>
#include <cfloat>
#include <cstdint>
#include <iomanip>
#include <iostream>
#include <map>
#include <set>
#include <vector>
#include <array>
#define REP(i, x) for(int (i) = 0; (i) < (x); (i) ++)
#define PREP(i, s, x) for(int (i) = (s); (i) < (x); (i) ++) 
#define MREP(i, s, x) for(int (i) = (s); (i) >= (x); (i) --)
#define IREP(i) for(int (i) = 0; ; (i) ++)
#define MOD 1000000007
#define PI (acos(-1))
// long long型の最大値:LLONG_MAX
// long long型の最小値:LLONG_MIN
// int型の最大値:INT_MAX
// int型の最小値:INT_MIN
// 絶対値:abs(n)
using namespace std;

int main(){
    long long a, b;
    cin >> a >> b;

    long long c;
    if(a > b){
        a = a ^ b;
        b = a ^ b;
        a = a ^ b;
    }

    if(b - 2 * a == 0){
        c = 3 * a;
    }else if(b - 3 * a == 0){
        c = 2 * a;
    }else if(2 * b - 3 * a == 0){
        c = b / 2;
    }else{
        c = -1;
    }
    cout << c << endl;
    return 0;
}