EGMO 2015, Problem 5. Let \(m, n\) be positive integers with \(m > 1\). Anastasia partitions the integers \(1, 2, \dots , 2m\) into \(m\) pairs. Boris then chooses one integer from each pair and finds the sum of these chosen integers. Prove that Anastasia can select the pairs so that Boris cannot make his sum equal to \(n.\)