#nullable enable

#region
var (_input, _iter) = (Array.Empty<string>(), 0);
T I<T>() where T : IParsable<T>
{
    while (_iter >= _input.Length) (_input, _iter) = (Console.ReadLine()!.Trim().Split(' '), 0);
    return T.Parse(_input[_iter++], null);
}
#endregion

var random = new Random();
var d = I<int>();

for (var i = 0; i < 3; i++)
{
    var z = random.Next(1, 30);
    var w = random.Next(1, 30);

    Console.WriteLine("? " + z);
    var y0 = I<double>();
    Console.WriteLine("? " + (z + w));
    var y1 = I<double>();
    Console.WriteLine("? " + (z + w + w));
    var y2 = I<double>();

    var d1 = (y1 - y0) / w;
    var d2 = (y2 - y1) / w;
    if (Math.Abs(d1 - d2) >= 1)
    {
        var ans = (Int128)Math.Max(d1, d2);
        Console.WriteLine("! " + ans);
        return;
    }
}
Console.WriteLine("! " + "NaN");