def series(n_terms=1000):
    """Estimate e with series: 1/1 + 1/1 + 1/(1*2) + 1/(1*2*3) + ..."""
    def factorial(n):
        result = 1
        for i in range(1, n+1):
            result *= i
        return result
    print(sum([1/factorial(i) for i in range(n_terms)]))