When he was in elementary school, his teacher once tried to keep the children busy, telling them to add up all the numbers from 1 to 100. Gauss did it quickly, like this: 1 + 100 = 101, 2 + 99 = 101, 3 + 98 = 101, and so on. There were a total of 50 pairs, so 50 × 101 = 5,050. The formula is formula_1. According to this archived website, the problem given to Gauss was actually more difficult to do.