using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
using System.Runtime.Intrinsics.Arm;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static long[] LList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => long.Parse(ReadLine())).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var gcd = GCD(c[0], c[1]);
        if (gcd > 1) WriteLine(-1);
        else
        {
            var dp = new bool[c[0] * c[1] + 1];
            for (var i = 0; i < c[1]; ++i) for (var j = c[0] * i; j < dp.Length; j += c[1]) dp[j] = true;
            var ans = 0;
            for (var i = 1; i < dp.Length; ++i) if (!dp[i]) ++ans;
            WriteLine(ans);
        }
    }
    static int GCD(int a, int b)
    {
        if (a < b) return GCD(b, a);
        if (a % b == 0) return b;
        return GCD(b, a % b);
    }
}