BigInteger Square Root in F#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Numerics;

namespace GUI
{
    public partial class SqrtForm : Form
    {
        private BigInteger original;  // The number to take the square root of.
        private BigInteger answer;    // To hold the answer to bigintSqrt(original)

        public SqrtForm()
        {
            InitializeComponent();
        }

        /// <summary>
        /// Resize and position the controls based on the news size of the window.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void SqrtForm_Resize(object sender, EventArgs e)
        {
            txtOriginalInt.Height = (this.Height - 150) / 2;
            sqrtButton.Location = new Point(sqrtButton.Location.X, txtOriginalInt.Location.Y + txtOriginalInt.Height + 15);
            lblAnswer.Location = new Point(lblAnswer.Location.X, sqrtButton.Location.Y + 44);
            txtSqrtAnswer.Location = new Point(txtSqrtAnswer.Location.X, sqrtButton.Location.Y + 63);
            txtSqrtAnswer.Height = txtOriginalInt.Height;  
        }

        /// <summary>
        /// Determine if text in the txtOriginalInt control can be parsed as a positive integer.
        /// </summary>
        /// <returns>True if the text is parsed a positive integer, else False.</returns>
        private bool IsParsedAsPositiveInt()
        {
            sqrtErrorProvider.Clear();
            bool notValid = ! BigInteger.TryParse(txtOriginalInt.Text, out original);
            if (notValid)
            {
                sqrtErrorProvider.SetError(txtOriginalInt, "Please enter a positive integer");
                txtSqrtAnswer.Text = string.Empty;
            }
            else
            {
                if (original.Sign == -1)
                {
                    notValid = true;
                    sqrtErrorProvider.SetError(txtOriginalInt, "Please enter a positive integer");
                    txtSqrtAnswer.Text = string.Empty;
                }
            }
            return ! notValid;
        }

        /// <summary>
        /// Call the F# bigintSqrt function and display the result.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void sqrtButton_Click(object sender, EventArgs e)
        {
            if (IsParsedAsPositiveInt())
            {
                // Call Sqrt
                original = BigInteger.Parse(txtOriginalInt.Text);
                answer = Roots.bigintSqrt(original);
                txtSqrtAnswer.Text = answer.ToString();
            }
        }
    }
}