√(2015*2016*2017*2018 + 1) = √(2015*2018 * 2017*2016 + 1) =
√(4066270*4066272 + 1) = √((4066271 - 1)*(4066271 + 1) + 1) = √(4066271² - 1 + 1) = √4066271² = 4066271
a² - b² = (a - b)(a + b)
