static int FrogRiverOne(int x, int[] a)
{
    var existing = new HashSet<int>();

    for (int k = 0; k < a.Length; k++)
    {
        var i = a[k];
        if (i <= x && existing.Add(i) && existing.Count == x)
        {
            return k;
        }
    }

    return -1;
}