import math


def A000108(i: int) -> int:
    """Catalan numbers: C(n) = binomial(2n,n)/(n+1) = (2n)!/(n!(n+1)!).

    Also called Segner numbers.
    """
    return (
        math.factorial(2 * i)
        // math.factorial(i)
        // math.factorial(2 * i - i)
        // (i + 1)
    )