syncing ipad with itunes without losing apps

Hello,

When I first bought my iPad, I apps from using iTunes on my laptop. Later on, I started directly downloading apps from the appstore on iPad.

But now I want to download apps using iTunes on my laptop, but when I want to sync I get a message that I may lose apps on my iPad. How can I sync again iTunes with my iPad without losing any of the apps I downloaded directly from the appstore on my iPad?

Thank you for your help.