item index generation and removal from ddl

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C#

ASP.NET

I am using dropdownlist. my code is shown below:

if (ddlDeposite.SelectedItem.Text == "Normal Savings")
        {
            int indexOfOnDepositForNormalSavings = -1;
            indexOfOnDepositForNormalSavings = ddlInterestCalculationPeriod.Items.IndexOf(ddlInterestCalculationPeriod.Items.FindByValue("5"));
            ddlInterestCalculationPeriod.Items.RemoveAt(indexOfOnDepositTypeForNormalSavings);

            int indexOfMaturity = -1;
            indexOfMaturity = ddlInterestCalculationPeriod.Items.IndexOf(ddlInterestCalculationPeriod.Items.FindByValue("6"));
            ddlInterestCalculationPeriod.Items.RemoveAt(indexOfMaturity);
        }
if (ddlDeposite.SelectedItem.Text == "Recurring Savings")
        {
            int indexOfOnDepositForRecuringSavings = -1;
            indexOfOnDepositTypeForRecuringSavings = ddlInterestCalculationPeriod.Items.IndexOf(ddlInterestCalculationPeriod.Items.FindByValue("5"));
            ddlInterestCalculationPeriod.Items.RemoveAt(indexOfOnDepositTypeForRecuringSavings);
        }

what i wanted is as user selects Normal Savings on ddlDeposite, cetain item of ddlInterestCalculationPeriod (say of index 5 and 6) is to be removed when user drops down on ddlInterestCalculationPeriod. similarly, as user selects Recurring Savings, certain it ddlInterestCalculationPeriod (say of index 5) is to be removed when user drops down on ddlInterestCalculationPeriod.. everything works fine when user selects normal savings or recurring savings on ddlDeposite for first time.. as user re selects normal savings or recurring savings on ddlDeposit, the index value ie indexOfOnDepositForNormalSavings, indexOfMaturity or indexOfOnDepositForRecuringSavings becomes -1. thus the error msg is

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

the event is selectedIndexChaged.. any help???