Calculate the Shipping cost

I want go give 50% discount in Shipping cost If customer buys two or more products.
and my problem is I couldn’t do that in woocommerce Flat rate Settings, I just tried (Shipping Cost)/[qty] its work if customer purchase two product only.. but if customer purchase more products the shipping cost keep on reducing.. how can I achieve my goal? is there any plugin or method to achieve this task?? Please help me ??