Alternating Series Sum

Let a₁ , a₂ be integers such that a₁ - a₂ + a₃ - a₄ + ........ + (-1)^n-1 a_n = n , for n ≥ 1. Then a₅₁ + a₅₂ + ........ + a₁₀₂₃ equals

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