I am not a great one for counting calories, but I know that 100 calories is not a lot.  It is 2,000 steps. Walking from one bus-stop to the next or using the stairs at work. Or half a bag of crisps or a small bar of chocolate.

J. O. Hill, H. R. Wyatt, G. W. Reed, and J. C. Peters (2003) report in the magazine Science that an increase in net expenditure of 100 calories a day can stop the increase in obesity in 90% of the population. Just walking an extra 2,000 steps, you might never gain another pound.