#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <array>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
#include <memory>
#include <regex>
using namespace std;

int main()
{
    int a, b;
    cin >> a >> b;
    if(b < a)
        swap(a, b);

    int c = -1;
    if(a % 2 == 0 && a * 3 == b * 2)
        c = a / 2;
    else if(a * 2 == b || a * 3 == b)
        c = a * 5 - b;
    cout << c << endl;

    return 0;
}