def sum_of_square_of_n_numbers(n):
    """
    The sum of squares of first n natural numbers
    ∑ n^2 = 1^2 + 2^2 + 3^2 +....+ n^2
    """
    try:
        n+1
    except TypeError: # invlid input hence return early
        return 
    if n < 0: # invlid input hence return early
        return 

    return n * (n + 1) * (2 * n + 1) // 6