What's the minimum amount of cAlories I should be consuming?

Hi, I'm trying to lose weight as quickly as possible but obviously I want the weight loss to be permeant and I want it done in a healthy way.

I exercise 4-6 times a week and do hour and half, mix between strength, cardio and classes. I lead quite an active life as in I'm out of bed every day 8am and up until 11/12. I walk everywhere, never get buses, the walk to the gym is a brisk 35 minutes each way etc. I work 3 days a week stacking shelves in a supermarket.

I'm 5,2 and I weigh 50kg atm. I'm really petite and small boned.

What's the minimum amount of calories I should consume a day without it going in to starvation mode.

And if I exercise should I be eating more? Taking my allowance up? because ATM I try to only eat 1400 max even if I've done loads of exercise