Quick calculation

1+2+3 + ... + 100=?

I read this in a book called Number Games by Ivan Moscovich, "A famous German Mathematician, Karl Fredrich Gauss ... When Gauss was a ten-year old student, his teacher wanted to keep his class busy for a long time so, he asked the students to add up all the number from 1 to 100 ... He realized that the first and last numbers (1 and 100) add up to 101. The second number (2) and the next-to-the-last number also add ed to 101 ... and so on ... He then had 100 pairs, which he multiplied by 101. Since Gauss only wanted the sum of one set of numbers from 1 to 100, he then divided the total in half."

the answer is 5050