Which of the following numbers is not a perfect square: $1^{2016}$, $2^{2017}$, $3^{2018}$, $4^{2019}$, or $5^{2020}$? The original answer is in the form $x^{y}$, please give the value of $x + y$.

Let's think step by step and output the final answer within \boxed{}.
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