How to get a flat stomach?

I am a naturally small person, I weigh 45kg so I don't want to lose weight necessarily. However i have a bit of a tummy which I don't like and I really want my stomach to be perfectly flat. I am in Year 12 so I don't have lots of time to go on regular runs or go to the gym.
Any suggestions would be brilliant, such as what exercises I could do, thank you! xx
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+1 y
Also, if you were to suggest I should do crunches or sit-up's etc. How many do you reckon I should do each night, and how long would I have to do that for before I actually noticed changes? :))
How to get a flat stomach?
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