The sum [math]a[/math] has 2020 terms, each of which is not bigger than [math]2020^{2020}.[/math] So, the whole sum is not bigger than [math]2020\cdot2020^{2020}[/math].
[math]a\leq2020^{2021}\leq2048^{2021}[/math]
[math]2048^{2021}=(2^{11})^{2021}=2^{11\cdot2021}=2^{22231}[/math]
[math]a\leq2^{22231}[/math]
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