Working out while sitting in office?

Dear ladies,
I had been searching for some exercise routines which can be done while sitting in office but didnt specifically find one here. So please bear with me if I am repeating an old thread.

Little back ground: I start my work at 7:45 in the morning (for which I have to wake up at least at 4:45 to be at work on time...long travel). Since then, my work keeps me really busy until I leave work at 3:45. I walk back to the train station, 2.5 km in 20 minutes. my BMI is in the normal range and I am not over weight.

BUT, the main problem is my tummy. I look like I am 4 months pregnant:spin But other parts like hands and legs are perfectly fine and look toned.

Now, my question is: is there any exercise I can do while I sit at work to get the muscles tightened in tummy? PLEASE! Help me. I want to give some exercise to my tummy while I am working. Please. Thank you!