static int Dominator(int[] a)
{
    var size = 0;
    var value = 0;
    var index = 0;

    for (int k = 0; k < a.Length; k++)
    {
        var v = a[k];
        if (size == 0)
        {
            size++;
            value = v;
            index = k;
        }
        else if (value != v)
        {
            size--;
        }
        else
        {
            size++;
        }
    }

    var candidate = size > 0 ? value : -1;
    var count = 0;
    foreach (var v in a)
    {
        if (v == candidate)
        {
            count++;
        }
    }

    if (count <= a.Length / 2.0)
    {
        index = -1;
    }

    return index;
}