CODE

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import decimal

def formulaFibWithDecimal(n):
    decimal.getcontext().prec = 3000000
    root_5 = decimal.Decimal(5).sqrt()
    phi = ((1 + root_5) / 2)

    a = ((phi ** n) - ((-phi) ** -n)) / root_5

    return round(a)

print(formulaFibWithDecimal(1000000000))