Leftovers are, quite literally, food that is "left over" from a previous meal. So the calories consumed in any leftover meal are essentially free, since you could have eaten that food the first time around, but chose not to, due to your superior dieting willpower.
Not only can you eat leftovers with net-zero calorie gain, but you can also eat as much of them as you want, based on this important dietary equation:
n x 0 = 0
Supplemental tip: Whenever you cook a meal, make way more than you can possibly eat, in order to provide plenty of zero-calorie food for future leftover meals.
Stay tuned for more important dieting tips!