How to convert string to datetime string str="14/09/2012 11:11 PM"

Hello guys,
Can anyone help me to know to convert string to Datetime
        string str="14/09/2012 11:11 PM";
I used parse excat
like this:
            string format = "yyyy/dd/MM HH:mm tt";

            DateTime MyDateTime = DateTime.ParseExact(comp1, format, CultureInfo.InvariantCulture);
 but this not converting exactly format which I have mentioned. It is working for mm/dd/yyy

so please help me soon anyone
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