def comb(n, m):
    """
    combination: nCm

    >>> comb(5, 2) # 5*4 // (2*1)
    10
    """
    c = d = 1
    if n >= m >= 0:
        while m > 0:
            c *= n
            d *= m
            n -= 1
            m -= 1
    return c//d