How can I generate N random values that sum to predetermined value?

I need your help with a little problem. I have four labels and I want to display on them random value between 0 to 100, and the sum of them must be 100.

This is my code :

private void randomly_Click(object sender, EventArgs e)
{
    double alpha = 0, beta = 0, gamma = 0, delta = 0;
    double temp;
    int tempDouble;

    Random rnd = new Random();

    alpha = rnd.Next(0, 100);

    temp = 100 - alpha;
    tempDouble = (int)temp;
    beta = rnd.Next(0, tempDouble);

    temp = 100 - (alpha + beta);
    tempDouble = (int)temp;
    gamma = rnd.Next(0, tempDouble);

    temp = 100 - (alpha + beta + gamma);
    tempDouble = (int)temp;
    delta = rnd.Next(0, tempDouble);

    temp = alpha + beta + delta + gamma;
    temp = 100 - temp;
    temp = temp / 4;

    alpha = alpha + temp;
    beta = beta + temp;
    gamma = gamma + temp;
    delta = delta + temp;

    cInsertion.Text = alpha.ToString();
    cMoyens.Text = beta.ToString();
    cInternational.Text = gamma.ToString();
    cRecherche.Text = delta.ToString();
}   

The problem is that I'm giving to the alpha the chance to have the biggest value, and for delta the lowest value.

Is there any way to give them all the same chance to have a real random value?