1516171819 15 16 17 18 19. 801 802 - 202 201 81 40 - 10 21. The next one is 15.
N 2first number last number sum where n is the number of integers.
For example n 4 in the last sum above. S 100 n 1 100 2 displaystyle S_100nfrac 11002. Minus the smaller consecutive numbers from the bigger ones. Another example-- we could start at 11.