What is the difference of using Invoke and without invoke for invoking c# delegate references

Question

Lets consider the below example,

Invoke the delegate reference with Invoke keyword:

public delegate double returndel(int r);

private void button1_Click(object sender, EventArgs e)
{
    returndel ret = CalculateArea;
    double yt = ret.Invoke(4);   
    MessageBox.Show(yt.ToString());
}

public double CalculateArea(int r)
{
    return 3.14 * r * r;
}

Invoke the delegate reference without Invoke keyword:

public delegate double returndel(int r);

private void button1_Click(object sender, EventArgs e)
{
    returndel ret = new returndel(CalculateArea);
    double yt = ret(4);
    MessageBox.Show(yt.ToString());
}

public double CalculateArea(int r)
{
    return 3.14 * r * r;
}

What is the difference between these two ?