On 06/15/2010 01:49 PM, superpollo wrote: > my solution: > > [...] > >>> print s > 536926141 Or, if you would like to use numpy: >>> import numpy >>> squares = numpy.arange(1, 2011, dtype=numpy.int)**2 >>> signs = numpy.ones(len(squares), dtype=numpy.int) >>> signs[3::5] = -1 >>> signs[4::5] = -1 >>> numpy.sum(signs*squares) 536926141 Cheers Andre