Bounds on the AM-GM gap?

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I have the expression $\frac{1}{2}(a_1+a_2) - \sqrt{a_1 a_2}$ and I need to upperbound it. Note that this is the "slack in the AM-GM inequality". In my specific case, $0\leq a_1,a_2 \leq 1$. Are there any useful bounds for this? I was not able to find anything relevant.