Given sum of 4 variables as a constant, how to maximize the sum of products?

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If variables a, b, c, and d are positive numbers and a + b + c + d = 1,

Can I find the maximum of the expression below?

ab + cd

and how can I reach that?

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Hint:$${(a+b+c+d)}^2-4ab-4cd={(a-b)}^2+{(c-d)}^2+2(ac+bd+ad+bc)\ge 0$$

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Hint: $ab+cd \leq (a+c)(b+d)$.