The expansion $1+x,1+x+x^2,1+x+x^2+x^3,\dots .1+x+x^2+\dots +x^{20}$ are multipled together and the terms of the product thus obtained are arranged in increasing powers of $x$ in the form of $a_0+a_{1}x+a_2{x}^2+\dots$, then,
Sum of coefficients of even powers of $x$ is

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